Periodic calibration on a communications channel

ABSTRACT

A method and apparatus for estimating the downlink signature for a remote transceiver which is part of a wireless communication system that includes a main transceiver for communicating with the remote transceiver. The main transceiver includes an array of transmit antenna elements. The method uses the remote transceiver for receiving signals when the main transceiver transmits downlink calibration signals. When the main transceiver also has a receive antenna array, the remote transceiver can transmit uplink calibration signals to the main transceiver for determining an uplink signature. The downlink and uplink signatures are used to determine a calibration function to account for differences in the apparatus chains that include the antenna elements of the arrays, and that enable downlink smart antenna processing weights to be determined from uplink smart antenna processing weights when the main transceiver includes means for smart antenna processing according to weights.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional PatentApplication Serial No. 60/083,875 for METHOD AND APPARATUS FORDETERMINING SPATIAL SIGNATURES WITH APPLICATION TO CALIBRATING A BASESTATION HAVING AN ANTENNA ARRAY to inventors Boros, Barratt, Uhlik, andTrott, Assignee ArrayComm, Inc., filed May 1, 1998.

FIELD OF INVENTION

[0002] This invention relates to the field of wireless communicationsystems, and more specifically, to a method and apparatus forcalibrating a communication station that includes an array of antennaelements.

BACKGROUND

[0003] Smart Antenna Systems

[0004] Antenna arrays may be used in any wireless communication receiveror transmitter or transceiver (herein under “communication station”)that transmits or receives radio frequency signals using an antenna orantennas. The use of antenna arrays in such a communication stationprovides for antenna performance improvements over the use of a singleelement antenna. These antenna performance improvements include improveddirectionality, signal to noise ratio, and interference rejection forreceived signals, and improved directionality, security, and reducedtransmit power requirements for transmitted signals. Antenna arrays maybe used for signal reception only, for signal transmission only, or forboth signal reception and transmission.

[0005] A typical application of antenna array communication stations isin a wireless communication system. Examples include a cellularcommunication system consisting of one or more communication stations,generally called base stations, each communicating with its subscriberunits, also called remote terminals and handsets. In cellular systems,the remote terminal may be mobile or in a fixed location, and whenfixed, such a system often is called a wireless local loop system. Theantenna array typically is at the base station. Terminology for thedirection of communication comes from conventional satellitecommunication, with the satellite replaced by the base station. Thus,communication from the remote terminal to the base station is called theuplink, and communication from the base station to the remote terminalis called the downlink. Thus, the base station antenna array transmitson the downlink direction and receives on the uplink direction. Antennaarrays also may be used in wireless communication systems to add spatialdivision multiple access (SDMA) capability, which is the ability tocommunicate with several users at a time over the same “conventional”(FDMA, TDMA or CDMA) channel. We have previously disclosed adaptivesmart antenna processing (including spatial processing) with antennaarrays to increase the spectrum efficiency of SDMA and non-SDMA systems.See Co-owned U.S. Pat. No. 5,515,378 for SPATIAL DIVISION MULTIPLEACCESS WIRELESS COMMUNICATION SYSTEM, U.S. Pat. No. 5,592,490 forSPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, U.S.Pat. No. 5,828,658 for SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESSCOMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING, and U.S. patentapplication Ser. No. 08/729,390 for METHOD AND APPARATUS FOR DECISIONDIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL PROCESSING.Systems that use antenna arrays to improve the efficiency ofcommunications and/or to provide SDMA sometimes are called smart antennasystems.

[0006] With smart antenna communication systems that use linear spatialprocessing for the adaptive smart antenna processing, during uplinkcommunications, one applies amplitude and phase adjustments in basebandto each of the signals received at the antenna array elements to select(i.e., preferentially receive) the signals of interest while minimizingany signals or noise not of interest—that is, the interference. Suchbaseband amplitude and phase adjustment can be described by a complexvalued weight, the receive weight, and the receive weights for allelements of the array can be described by a complex valued vector, thereceive weight vector. Similarly, the downlink signal is processed byadjusting the amplitude and phase of the baseband signals that aretransmitted by each of the antennas of the antenna array. Such amplitudeand phase control can be described by a complex valued weight, thetransmit weight, and the weights for all elements of the array by acomplex valued vector, the transmit weight vector. In some systems, thereceive (and/or transmit) weights include temporal processing, and thenare called spatio-temporal parameters for spatio-temporal processing. Insuch cases, the receive (and/or transmit) weights may be functions offrequency and applied in the frequency domain or, equivalently,functions of time applied as convolution kernels. Alternatively, eachconvolution kernel, if for sampled signals, may itself be described by aset of complex numbers, so that the vector of convolution kernels may bere-written as a complex values weight vector, which, for the case ofthere being M antennas and each kernel having K entries, would be avector of KM entries.

[0007] The receive spatial signature characterizes how the base stationarray receives signals from a particular subscriber unit in the absenceof any interference or other subscriber units. A receive weight vectorfor a particular user may be determined using different techniques. Forexample, it may be determined from spatial signatures. It also may bedetermined from the uplink signals received at the antennas of the arrayfrom that remote user using some knowledge about these uplink signals,for example, the type of modulation used. The transmit spatial signatureof a particular user characterizes how the remote user receives signalsfrom the base station in the absence of any interference. The transmitweight vector used to communicate on the downlink with a particular useris determined either from the receive weight vector (see below under“The Need for Calibration”) or from the transmit spatial signature ofthe particular user and the transmit spatial signatures of the otherusers in such a way as to maximize the energy to the particular user andminimize the energy to the other users.

[0008] U.S. Pat. No. 5,592,490 for SPECTRALLY EFFICIENT HIGH CAPACITYWIRELESS COMMUNICATION SYSTEMS describes spatial signatures and theiruses, and U.S. Pat. No. 5,828,658 for SPECTRALLY EFFICIENT HIGH CAPACITYWIRELESS COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING,incorporated herein by reference, describes how to extend this tospatio-temporal processing using spatio-temporal signatures.

[0009] Thus, while the description herein is provided in terms ofspatial signatures, adding time equalization to provide spatio-temporalprocessing is easily accommodated, for example by adding the concepts ofspatio-temporal signatures, which may be described by MK vectors (bothuplink and downlink) when the temporal processing is using equalizerswith K taps (i.e., convolution kernels of length K in the weightconvolving functions). Thus, how to modify the invention to accommodatespatio-temporal processing and spatio-temporal signatures would be clearto those of ordinary skill in the art, for example in view ofabove-referenced and incorporated herein by reference U.S. Pat. No.5,828,658. Therefore, those in the art would understand that any timethe term spatial signature is used, this might indeed be referring to aspatio-temporal signature in the context that the invention is beingapplied to a communication station equipped with means forspatio-temporal processing.

[0010] The Need for Calibration

[0011] It is desirable to determine the transmit weight vector from thereceive weight vector for a particular user. More generally, it isdesirable to determine the appropriate transmit signals to use fortransmitting to a particular user from signals received from that user.Practical problems may make difficult determining the transmit weightvector from the receive weight vector for a particular user. Frequencydivision duplex (FDD) systems are those in which uplink and downlinkcommunications with a particular remote user occur at the differentfrequencies. Time division duplex (TDD) systems are those in whichuplink and downlink communications with a particular remote user occurat the same frequency but in different time slots. In a TDD system,because of the well known principle of reciprocity, it might be expectedthat determining the transmit weight vector from the receive weightvector is straightforward. However, on the uplink, the received signalsthat are being processed may be somewhat distorted by the receiveelectronics (the receive apparatus chains) associated with each of theantenna elements of the antenna array. The receive electronics chainincludes the antenna element, cables, filters, RF receivers and othercomponents, physical connections, and analog-to-digital converter(“ADC”) if processing is digital. In the case of a multi-element antennaarray, there typically is a separate receive electronics apparatus chainfor each antenna array element, and thus the amplitude and the phase ofeach of the received signals at each element may be distorteddifferently by each of the receive apparatus chains. In addition, thereare RF propagation effects that take place on the uplink between thesubscriber unit and a particular receiving antenna, such effectsincluding without limitation the path loss, fading and shading effects,multipath, and near-field scattering, and these effects may be differentfrom antenna element to antenna element. Note that the receiveelectronics chain and the RF propagation effects together make up theuplink spatial signature for the remote user. A receive weight vectorthat does not take these receive electronics chain and RF propagationeffects into account will be in error, causing less than optimalreception at the base station. However, in practice, communication maystill be possible. Also, when a receive weight vector is determinedusing some knowledge of the characteristics of the received signal, forexample, the type of modulation used, such a method already takes intoaccount the uplink receive electronics chain and RF propagation effects.When one transmits downlink signals through the antenna array, each ofthe signals radiated by an antenna element goes through a differenttransmit electronics apparatus chain; thus possibly causing differentamplitude and phase shifts in the transmitted signals. In addition,there are again RF propagation effects. If the transmit weight vectorwas derived from a receive weight vector that did not take thedifferences in the receive electronics chains and RF propagation intoaccount, transmission from the base station may be hard to achieve.Further difficulty may result if the transmit weight vector does nottake differences in the transmit electronics chains and transmit RFpropagation effects into account, possibly making communication usingsuch a transmit weight vector impossible.

[0012] The purpose of calibration is to determine calibration factorsfor compensating for the different amplitude and phase errors that occurin the signals in the receive chain and uplink RF propagation, and thedifferent amplitude and phase errors that occur in the transmit chainand downlink RF propagation, the calibration factors used in acommunication station to determine a transmit weight vector fortransmitting to a remote user from the set of signals received from theremote user. It should be added that because the phase and amplitudeshifts that occur in the receive and transmit apparatus chains are, ingeneral, frequency dependent, so in general are the calibration factorsfrequency dependent.

[0013] In the case of a TDD system, the uplink and downlink RFpropagation effects cancel so that the calibration factors areindependent of the location of the subscriber unit.

[0014] It is known that compensation can be achieved by convolving eachof the M signals received or transmitted by the antenna elements by acalibration function ( i.e., by a complex valued time sequence), whereeach calibration function describes the transfer function correctionrequired to compensate for the gain and phase errors a signal undergoeswhen passing through the transmit and receive apparatus chains. In somesystems, this can be simplified to multiplicative correction, where eachcalibration function is a calibration factor—a complex valued numberthat describes the required amplitude and phase correction required forcompensation. In general, the set of calibration functions defines acalibration vector function with each element a calibration function. Inthe case of multiplicative correction, the set of calibration factorsdefines a calibration vector with each element a calibration factor.

[0015] Determining the transmit weight vectors from the receive weightvectors for a particular user is more difficult in the case of an FDDsystem because reciprocity may no longer be assumed. One needs toadditionally take into account the differences in propagation on theuplink and downlink. Once one does take such differences into account,there still is a need to determine calibration factors for compensatingfor the different amplitude and phase errors that occur in the signalsin the receive chain and uplink RF propagation and the differentamplitude and phase errors that occur in the transmit chain and downlinkRF propagation. In general, single calibration factors that areindependent of the location of the remote user may not be possible. Insuch a case, one needs to be able to determine the uplink and downlinkspatial signatures.

[0016] In the case of no calibration factors that are independent of theremote user location being possible, when there is some functionalrelationship that enables one to determine the transmit weight vector touse from the received signals and some parameter, for example, the angleof arrival, there still is a need to determine a set of calibrationfunctions for compensating for the different amplitude and phase errorsthat occur in the signals in the receive chain and uplink RF propagationand the different amplitude and phase errors that occur in the transmitchain and downlink RF propagation, these functions being dependent onone or more parameters of the remote user, for example the angle ofarrival.

[0017] The Need for Signature Estimation

[0018] When no simple calibration (as defined above) is possible, onestill needs to compensate for the different amplitude and phase errorsthat occur in the signals in the receive chain and uplink RFpropagation, and the different amplitude and phase errors that occur inthe transmit chain and downlink RF propagation. The purpose of signatureestimation is to determine the uplink and downlink spatial signatureswhich characterize these differences. Thus calibration is a special caseof signature estimation when either 1) the RF propagation effects cancelso that downlink weights can be determined from uplink signals orweights, or 2) there is some simple functional relationship of the RFpropagation effects so that uplink weights can be determined from uplinksignals and some parameters of the remote user, for example, the angleof arrival of the uplink signals.

[0019] Other Methods

[0020] Known methods for determining array calibrations each have one ormore associated drawbacks. Most known methods require external measuringequipment which may be expensive, unwieldy and cumbersome to userepeatedly. Secondly, conventional calibration methods are sensitive todrifts in system parameters, such as frequency references, over theextended period of time during which measurements are being taken, andthese drifts result in inaccuracies in the measured array calibrations.In addition, some known techniques only determine multiplicative ratherthan convolution kernel calibrations despite the need to calibratefrequency dependent components in the antenna array. In order toeliminate this frequency dependence and still use multiplicativecalibrations, it is necessary to calibrate the antenna array for eachfrequency channel of communication. Thirdly, the transfercharacteristics of the RF electronics depend on changing ambientconditions such as temperature and humidity which make it essential thatantenna arrays be repeatedly calibrated in their ambient environment.

[0021] Harrison et al. disclose in U.S. Pat. No. 5,274,844 (Dec. 28,1993) a method for calibrating transmit and, separately, receive chains(as complex valued vector transfer functions) in two experiments whichinvolve a data bus connecting a resource controller to a remoteterminal. In the first experiment, the data bus indicates to the remoteterminal to send a known signal to the base station. This determines thereceive apparatus chain calibration. In a second experiment, the signalsreceived at the remote terminal are sent back to the resource controllervia the data bus to enable determining the transmit apparatus chaincalibration.

[0022] Co-owned U.S. Pat. No. 5,546,090, issued Aug. 13, 1996, andassigned to the assignee of the present invention, discloses acalibration method which can determine both transmit and receivecalibrations using a simple transponder co-located with the remoteterminal that retransmits to the base station the signals received atthe remote terminal from the base station. Such a method does notrequire the wired data-bus of the Harrison et al. invention. Still,additional transponder equipment is required.

[0023] PCT Patent application publication WO 95/34103 (published Dec.14, 1995) entitled ANTENNA ARRAY CALIBRATION, Johannisson, et al.,inventors, discloses a method and apparatus for calibrating thetransmission (and reception) of an antenna array. For transmitcalibration, an input transmit signal is inputted into each antennaelement one antenna at a time. After the input transmit signal haspassed through a respective power amplifier, the signal transmitted byeach antenna element is sampled by a calibration network. The resultingsignal is fed into a receiver, and a computation means relates thereceived signal with the original transmit signal for each antennaelement. Correction factors can then be formed for each antenna element.The antenna elements may then be adjusted (in amplitude and phase, orin-phase I and quadrature Q components) using the correction factors soas to ensure that each element is properly calibrated duringtransmission. For receive calibration, a known input signal is generatedand injected using a calibration network (a passive distributionnetwork) into each antenna element of the antenna array. The signalspass from the antenna elements through respective low noise amplifiers,and the signals thus received by each antenna element are measured by abeam forming apparatus. The beam forming apparatus can then generatecorrection factors by comparing the injected signal with the measuredsignals so as to individually calibrate each antenna element. Thecorrection can be described as amplitude and phase corrections, or ascorrections in in-phase I and quadrature Q components.

[0024] U.S. Pat. No. 5,530,449 to Wachs et al. entitled PHASED ARRAYANTENNA MANAGEMENT SYSTEM AND CALIBRATION METHOD (herein under “Wachs”)describes a management system and calibration method for use with aphased array antenna that employs a system level measurement ofamplitude and phase, conducted during nodal operation, to determine onan element by element basis, the tracking performance of individualchains for the antennas. The system and method measure the amplitude andphase of individual element chains utilizing probe carriers. Therequired correction coefficients for each chain are determined from themeasured amplitude and phase data, and each individual element chain isindividually compensated to remedy the amplitude and phase errors. Thesystem separately calibrates forward and return link phased arrayantennas on a phased array antenna communication station which is on asatellite. In one embodiment, a separate remote calibration station isused. For calibrating the transmit paths, the probe signal istransmitted to an antenna at the calibration system alternatively fromone element (a reference element) and an element under test. The signalsreceived at the calibration station are compared to determine thecorrections. A separate communication link also is used to providecommunication between the calibration station and the satellite. In thereceive direction, the remote calibration station is used to transmit toall antenna elements of the phased array, but only two elements arealternately sampled to form the calibration carrier. The calibrationcarrier is then downlinked at Ka band to a gateway hub station forcomputation. In an alternate embodiment, a local sense antenna at thesatellite's communication station is used to sample outputs of thetransmit antenna elements. In both embodiments, separate calibrationsare carried out for receive and transmit paths, and extra equipment isneeded, either a separate remote calibration station, with an additionallink, or a separate sense antenna system. Several features of Wachs'system are of note. First, additional hardware is required in the formof a separate calibration station or probe antenna. Second, specialwaveforms need to be used for that calibration, rather than ordinarycommunication waveforms supported by standard air interfaces. This meansthat the communication station needs additional hardware for forming andtransmitting such waveforms, and the calibration station needs specialreceiving/demodulating hardware, and cannot reuse standard hardware.Thus there is a chance that a Wachs-like system adapted for use in awireless communication system may not be allowed to operate in somecountries.

[0025] Thus these known methods provide separate calibrations for thereceive and transmit paths. The methods require special calibrationapparatus. Some known methods and systems use special waveforms, andthus need additional hardware for processing such waveforms, and also donot conform to any established air interface standards, so face the riskof not being allowed to operate in some countries Those known systemsthat also calibrate for the different air paths between the base stationantenna elements and the subscriber unit are more properly classified asspatial signature estimating techniques under the definition ofcalibration used herein.

[0026] Parish et al. in co-owned U.S. patent application Ser. No.08/948,772 for METHOD AND APPARATUS FOR CALIBRATING A WIRELESSCOMMUNICATION STATION HAVING AN ANTENNA ARRAY, describe a calibrationmethod for a base station with an array of antenna elements that doesnot require any additional calibration apparatus. One aspect includestransmitting a prescribed signal from each antenna element using thetransmit electronics of that antenna element while receiving thetransmitted signal in at least one of the receiver electronics chainsnot associated with the antenna. This is repeated, transmittingprescribed signals from other antenna elements using other transmitapparatus chains until prescribed signals have been transmitted from allantenna elements for which calibration factors are required. Calibrationfactors for each antenna element are determined as a function of theassociated transmit electronics chain and receiver electronics chaintransfer functions. When downlink and uplink communication occurs in thesame frequency channel, a single calibration factor is determined forany antenna element. In one version of the Parish et al. invention, thesingle calibration factor is in phase a function of the differencebetween the transmit apparatus chain transfer function phase and thereceiver apparatus chain transfer function phase associated with aparticular antenna element. In another aspect of the Parish et al.invention, the calibration factors so determined are used fordetermining a set of transmit weights from a set of receive weights.

[0027] While the Parish et al. invention enables determining a singleset of calibration factors for the base station which enables a downlinkset of weights to be determined from an uplink set of weights withoutrequiring some additional apparatus such as a transponder, andcalibrates for differences in base station electronics paths, the Parishet al. method cannot be adapted to estimate spatial signatures to dealwith RF propagation path differences which may occur. In addition, thebase station needs to enter a spatial calibration mode for carrying outthe calibration experiment, and thus cannot be used for any otherpurpose during that time.

[0028] Also, there is no mention in the prior art of the capability ofcalibrating by combining measurements from a plurality of remotetransceivers.

[0029] Desirable Features

[0030] The main purpose of the calibration process is to acquirecalibration information for the base station. This may involve measuringthe gain and phase differences between the uplink and downlink channels.Accuracy and high precision are of great importance during thisprocedure. If the calibration information is not accurate, then the beampattern on the downlink will be highly distorted. As a consequence, lessenergy will be radiated toward the target user, and an excess amount ofinterference will be radiated toward co-channel users. This will have anegative effect on the downlink signal quality and on the downlinkrange. Ultimately, a bad calibration strategy may significantly reducethe capacity of the wireless network.

[0031] One desirable feature of a calibration method is that only a basestation and a subscriber unit are needed for calibration with no furtherequipment such as signal generators, transponders, calibration stations,additional antennas, probes, or other equipment, being required. Such asystem ideally should be able to calibrate for differences in both thereceive and transmit electronics. Such systems also should use ordinarycommunication waveforms substantially conforming to the particular airinterface standard of the wireless communication system in which theyoperate. This enables reusing standard hardware, and also ensuresnon-violation of standards and maintaining compatibility with any futuremodifications with standards. By “conforming to an air interfacestandard” we mean conforming to the channel structure and modulation ofan air interface, where “channel structure” is a frequency slot in thecase of FDMA, a time and frequency slot in the case of TDMA, and a codechannel in the case of CDMA, and “modulation” is the particularmodulation scheme specified in the standard.

[0032] Another desirable feature is that the method can be used forsignature estimation in order to also account for differences in the RFpaths.

[0033] Another desirable feature of a calibration method is ease of useand the ability to carry out the calibration rapidly and frequently,even for example, as frequently as several times a minute. Thisultimately increases the downlink processing accuracy which has aprofound effect on signal quality, capacity, coverage, and possiblyother parameters.

[0034] Another desirable feature of a calibration method is that eachand every subscriber unit supports calibration.

[0035] Another desirable feature for a calibration system is the abilityto carry out some or all of the processing of received data forcalibration within the subscriber unit, thus not requiring thesubscriber unit to send the received data back to the base station andnot requiring the base station to carry out all of the processing. Thecomputational burden of the base station thus may be significantlyreduced by “distributing” the load across intelligent subscriber units.This feature is particularly desirable, for example, for base stationsthat service many subscriber units, or that calibrate before each callor even several times during each call.

[0036] Another desirable feature is the ability to initiate calibrationon any available conventional channel on the base station, for example,any carrier and any time slot of a FDMA/TDMA system. This furtherenhances flexibility since one can choose any timeslot and any carrierwhich is available for use at the moment.

[0037] Another desirable characteristic for a calibration method is theability to calibrate a base station without having to take the basestation off-line for calibration, thus enabling base station calibrationto be performed while the base station services hundreds of calls, forexample, in a FDMA/TDMA/SDMA system on other carriers (frequencyslots)/timeslots/spatial channels. This feature is especially importantfor wideband base stations that service many conventional channels(e.g., carriers for an FDMA/TDMA system) at the same time.

[0038] Another desirable characteristic for a calibration method is theability carry out rapid calibration even several times during anexisting call.

[0039] Another desirable characteristic for a calibration method is theability to carry out calibration in a seamless manner during an ongoingcall so that a base station may be able to continuously calibrate itselfduring some calls.

[0040] Another desirable characteristic for a calibration method is theability to carry out calibration with several remote transceivers bycombining measurements, each of which may be able to “see” only a subsetof a communication station's antenna array, or each of which may face adifferent interference environment.

[0041] Another desirable characteristic for a calibration method is theability to determine whether calibration is accurate, for example byperforming statistical measurements, together with the ability to feedback such information to the communication station to determine, forexample, if the combining from several remote stations may be necessary.

[0042] Another desirable feature is high accuracy, with immunity tofrequency offset, timing misalignment, I/Q mismatch, and phase noisethat typically might occur in communication with inexpensive subscriberunits.

[0043] Thus there still is a need in the art for a calibration methodand apparatus that include all or most of the above characteristics. Forexample, the is a need for a system and method one that are accurate andsimple, both in terms of the equipment necessary and the time required,so that calibration can be performed repeatedly and rapidly wherever andwhenever desired. There also is a need in the art for a simplecalibration technique that only uses existing base station electronicsand does not require special calibration hardware. There also is a needin the art for a method that enables one to determine transmit weightvectors from receive weight vectors, including calibrating for thereceive electronics and transmit electronics, the calibration obtainedusing simple techniques that use existing base station and subscriberunit electronics and do not require special calibration hardware.

[0044] Thus there still is a need in the art for efficient methods thatdetermine uplink spatial signatures for correcting for the differencesin uplink RF paths and receive electronics and downlink spatialsignatures for correcting for the differences in downlink RF paths andtransmit electronics.

SUMMARY

[0045] An feature of the present invention is enabling calibrating acommunication station having an antenna array for differences inelectronics paths, the calibration using only the communication stationand a subscriber unit.

[0046] Another feature of the invention is providing calibration thatenables using a calibrated transmit weight vector, the transmit weightvector essentially determined from a receive weight vector, thecalibration taking into account differences in electronics paths.

[0047] Another feature of the invention is determining spatialsignatures that enable using a calibrated transmit weight vector, thetransmit weight vector essentially determined from a receive weightvector, the calibrating taking into account differences in electronicspaths and RF propagation paths.

[0048] Another feature of the invention is enabling determining theuplink spatial signature of a subscriber unit communicating with acommunication station, the determining using only the communicationstation and the subscriber unit.

[0049] Another feature of the invention is enabling determining thedownlink spatial signature of a subscriber unit communicating with acommunication station, the determining using only the communicationstation and the subscriber unit.

[0050] Still another feature of the invention is calibrating acommunication station having an antenna array that the calibrating easyand without taking the communication station off the air for thoseconventional channels not currently being calibrated.

[0051] Still another feature of the invention is calibrating acommunication station having an antenna array, the calibrating able tobe carried out partially or in total at a subscriber unit.

[0052] Still another feature of the invention is calibrating acommunication station, the calibrating method providing high accuracy,with immunity to frequency offset, timing misalignment, I/Q mismatch,and phase noise that typically might occur in communicating withinexpensive subscriber units.

[0053] Another feature of the invention is providing a calibrationmethod and apparatus that can be readily implemented in a radiofrequency system and that make it practical to perform frequent androutine system calibration, the calibration enabling the use of acalibrated transmit weight vector, the transmit weight vectoressentially determined from a receive weight vector, the calibrationincluding correcting for differences in electronic paths and fordifferences in RF propagation effects.

[0054] Yet another feature is enabling rapid calibration even severaltimes during an existing call.

[0055] Yet another feature is enabling carrying out calibration in aseamless manner during an ongoing call so that a communication stationmay be able to continuously calibrate itself during a particular call.

[0056] Yet another feature is the ability to carry out calibration withseveral remote transceivers by combining measurements, each of which maybe able to “see” only a subset of a communication station's antennaarray, or each of which may face a different interference environment.

[0057] Yet another feature is providing the ability to determine whethercalibration is accurate, for example by performing statisticalmeasurements, together with the ability to feedback such information tothe communication station to determine, for example, if the combiningfrom several remote stations may be necessary.

[0058] These and other features will become clear from reading thedetailed description of the preferred embodiments of the inventionprovided herein below

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The present invention will be more fully understood from thedetailed description of the preferred and some alternate embodiments ofthe invention, which, however, should not be taken to limit theinvention to any specific embodiments, but are for explanation andbetter understanding only. The embodiments in turn are explained withthe aid of the following figures:

[0060]FIG. 1 shows the uplink and downlink signal flow on the basestation;

[0061]FIG. 2 shows the decomposition of the uplink and downlink channelsinto “propagation” and “electronic” factors;

[0062]FIG. 3 illustrates the frame structure of a typical TDD system;

[0063]FIG. 4 shows the receive signal processor and the uplink weightcomputation;

[0064]FIG. 5 illustrates the symmetry between the uplink and downlinksignal paths;

[0065]FIG. 6 shows the internal structure of the transmit weightgenerator;

[0066]FIG. 7 shows the protocol sequence during calibration;

[0067]FIG. 8 illustrates the decomposition of a 6-element circular arrayinto 2-element subarrays;

[0068]FIG. 9 illustrates the uplink signature estimation at the basestation;

[0069]FIG. 10 shows the downlink signature estimation at the subscriberunit;

[0070]FIG. 11 shows a flowchart of one embodiment of a method forcarrying out downlink signature determination with calibration burstsinterspersed with normal TCH bursts;

[0071]FIG. 12 shows the architecture of a typical subscriber unit inwhich aspects of the present invention may be implemented;

[0072]FIG. 13 shows the results of testing a two antenna elementimplementation of the method for downlink signature estimation;

[0073]FIG. 14 shows the results of testing an implementation of themethod for downlink signature estimation using a single transmitter andantenna element; and

[0074]FIG. 15 shows the results of testing an implementation of themethod for downlink signature estimation using a single transmitter andantenna element, but with a different set of frequencies than used toobtain the results of FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0075] A Note on Reference Numerals

[0076] The first one or two digits in a reference numeral indicate onwhich figure that reference numeral is first introduced. Referencenumerals between 100 and 199 are first introduced in FIG. 1, thosebetween 200 and 299 are first introduced in FIG. 2, and so forth. Forexample, reference numeral 111 is first introduced in FIG. 1, 909 isfirst introduced in FIG. 9, 1009 is first introduced in FIG. 10, and1211 is first introduced in FIG. 12.

[0077] General System Description

[0078] The invention preferably is implemented in wireless cellularcommunication systems which include a base station (i.e., a transceiver,a communications station) with a multiple antenna array that uses smartantenna techniques for uplink or downlink communication or both. Thepreferred implementation is in a system that operates using the PersonalHandyphone (PHS) air interface communication protocol. Twoimplementations are one in which the subscriber units are fixed inlocation, and the other in which subscriber units may be mobile.Above-mentioned and incorporated-herein-by-reference co-owned U.S.patent application Ser. No. 08/729,390 describes the hardware of a basestation of a mobile system in detail, the base station preferably havingfour antenna elements. While the invention is useful for mobile andfixed subscriber unit situations, details are provided herein forincorporating the invention into a system with fixed location subscriberunits. Wireless systems with fixed locations are sometimes calledwireless local loop (WLL) systems. A WLL base station into which someaspects of the present invention are incorporated is described in U.S.patent application Ser. No. 09/020,049 for POWER CONTROL WITH SIGNALQUALUTY ESTIMATION FOR SMART ANTENNA COMMUNICATION SYSTEMS,incorporated-herein-by-reference, while the subscriber unit for use insuch a WLL system is described in U.S. patent application Ser. No.08/907,594 for METHOD AND SYSTEM FOR RAPID INITIAL CONTROL SIGNALDETECTION IN A WIRELESS COMMUNICATION SYSTEM. The WLL base stationdescribed in above-referenced U.S. patent application Ser. No.09/020,049 includes SDMA and may have any number of antenna elements,and many of the simulations described herein will assume a six-antennaarray. It will be clear to those of ordinary skill in the art that theinvention may be implemented in any smart antenna based system using anyair interface with one or more than one spatial channel(s) perconventional channel, and having mobile, fixed, or a combination ofmobile and fixed subscriber units. Such a system may be analog ordigital, and may use frequency division multiple access (FDMA), codedivision multiple access (CDMA), or time division multiple access (TDMA)techniques, the latter usually in combination with FDMA (TDMA/FDMA).

[0079] Note that while the preferred embodiment is to apply theinvention in a wireless communication system having base stations, eachbase station having subscriber units, the invention also is applicableto peer to peer communication from one radio to another. There is noinherent need to define the concept of a base station or subscriberunit, and how to modify this description to accommodate the peer-to-peercase would be clear to one of ordinary skill in the art. Therefore,while the invention is described as being implemented in a communicationstation and a subscriber unit, the communication station in this contextmay be any radio transceiver equipped with an antenna array, and thesubscriber unit may be any other radio transceiver remote to thearray-equipped transceiver and able to communicate with thearray-equipped transceiver using some modulation scheme. While thepreferred embodiment describes a base station that has a single arrayfor both uplink (receive) processing and downlink (transmit) processing,with means for adaptive smart antenna processing on the uplink and thedownlink, the invention also is applicable to a base station that has anarray only for transmit processing, and for a base station that uses aseparate antenna array for uplink processing and for downlinkprocessing. When only a single antenna is used for receiving signals,the calibration factor is the downlink signature since all receivedsignals pass through the same receive electronics chain. Also, the“number” of antenna is clearly the number of “active” antennas, that is,the number of antenna used for communication.

[0080] While the calibration is intended in the embodiments describedherein for use in adaptive smart antenna processing, the calibration maybe for any other purpose, so that the antenna-array-equipped transceiverneed not even include means for adaptive smart antenna processing.

[0081]FIG. 1 depicts the uplink and downlink signal flow through atypical base station (BS) on which the present invention may beembodied. Base station 101 includes an array of antenna elements 105.The base station communicates with one or more subscriber units such assubscriber unit 141 and subscriber unit 143. In the preferred embodimentthe base station has a single array of antenna elements that are usedfor both receive and transmit, so that a receive/transmit unit 107 isused. For frequency domain duplexing unit 107 is a frequency duplexerand for time domain duplexing (TDD), such as used in the preferredembodiment, unit 107 is a switch. On the downlink, signals from thesubscriber units are received at the antenna array. Those signals 106pass through the switch 107 set to the receive position and thesesignals pass through the receive RF electronics 109. In thisdescription, all the characteristics of the receive RF electronics,including all the cables and the switch characteristics and the RFreceivers, and other receive paths, are all lumped together. The receiveRF electronic unit 109 converts the RF signals to baseband signals 110.In the preferred embodiment, receive RF electronics unit 109 includesanalog RF components, including analog downconversion, analog to digitalconverters, and digital downconverter components to produce digitalbaseband antenna signals 110, and these baseband received antennasignals are processed by receive signal processor 111 to generate areceived signal from a particular subscriber unit, for examplesubscriber unit 141. The receive signal processor includes determining aweighted sum of the complex valued (in phase I and quadrature Q) antennasignals in an optimal manner where the weighting is in amplitude andphase, and where optimal means that the desired signal components areenhanced by a maximum amount and the non-desired components aresuppressed by a maximum amount.

[0082] The complex valued receive weights are computed by locking onto aknown training sequence, or by using some decision-directed technique,or “blindly” by using some other special structure in the signal. Ingeneral, it is not essential to know the phase and amplitude relationsof the receive electronics in order to perform the computation of theuplink (i.e. receive) weights. See below and in above-referencedco-owned U.S. patent application Ser. No. 08/1729,390 filed Oct. 11,1996 for more details on how these weights are computed.

[0083]FIG. 1 shows the output of the receiver part of the base stationas being voice or data 113 with signals which are directed to theNetwork Interface Unit (NIU). Thus, as shown in FIG. 1, receive signalprocessor 111 also includes all the demodulation function.

[0084] On the downlink the base station receives voice/data from the NIUdenoted 121 in FIG. 1. The signal is modulated according to the systemspecification. A transmit signal processor 123 includes distributingcomplex valued weighted copies 124 of the modulated baseband signal (theweighting according to a set of complex valued transmit weights), andthe weighted transmit antenna signals are fed to transmit RF electronicsunit 125 to generate a set of RF transmit signals 127, one signal aimedat each antenna element of antenna array 105. These RF antenna signalsare fed to the corresponding antenna array element through TX/RX switch107 which is set in the transmit position. The transmit weights arechosen so that the antenna array radiates most of the energy towards aparticular subscriber unit (“beam-forming”) and it transmits minimalenergy toward co-channel users (“null-placing”). In the preferredembodiment the set of transmit weights 118 is computed directly from theset of receive weights 115 generated by receive signal processor 111,and the computation is carried out by transmit weight generator 117 inreal time. However, during this computation the transmit weightgenerator 117 must take into account the gain and phase differencesbetween the uplink and downlink propagation channels where the channelsinclude both the air path from and to a subscriber unit and thevariation among the different signal parts within the receive RFelectronics and also within the transmit RF electronics. In thepreferred embodiment this information is stored in calibration storageunit 131 in the form of a calibration vector 133 as will be describedbelow. Determining this calibration information is the main goal of thepresent invention.

[0085] Uplink and Downlink Signal Path Descriptions

[0086] In this description, the number of elements in the base stationantenna array 105 shall be denoted by M. Thus, on the uplink there are Msignal paths from a subscriber unit, one to each of the M inputs of thereceive signal processor 111. Similarly, on the downlink, there are Msignal paths, one from each of the M inputs of transmit signal processor123 to the subscriber unit. Each of these signal paths is describedherein by a complex valued number that characterizes the phase andamplitude distortion of a baseband signal. As a compact representation,in this description, the uplink and downlink channels thus aremathematically described by M-dimensional complex valued vectors denoteda_(rx) and a_(tx), respectively, where M is the number of elements inthe base-station antenna array 105, and where each element in the vectorrepresents the path associated with one of the antenna elements in array105. Such a description is particularly accurate when the differences inpropagation times from (or to) a remote subscriber unit and individualantenna elements (delay spread) are much smaller than the symbol periodfor a system that uses a digital modulation scheme, such as the systemof the preferred embodiment. The vectors a_(rx) and a_(tx) may berecognized as the (un-normalized) uplink spatial signature and downlinkspatial signature, respectively, for the subscriber unit for this basestation.

[0087] Throughout the description, the uplink and downlink signatures,and the uplink and downlink weights, will all be described in baseband.It would be clear to those of ordinary skill in the art that theadaptive smart antenna processing, including any weighting in amplitudeand phase, may alternatively be carried out in some other band, forexample, in intermediate frequency or in the passband. In such a case,the signature and all its components similarly would be defined in thatfrequency band.

[0088] The main goal of the invention is to calibrate the base station.Assuming identical RF propagation on the uplink and downlink, a singlesubscriber unit can be used together with its base station to carry outthe calibration. It also will be apparent that the method enables theseparate determination of the uplink and downlink signatures for anysubscriber unit. The ease with which such data can be obtained enablesone to obtain complete signature information for any (and even every)active subscriber unit. Therefore, in addition to calibrating the basestation by running a simple calibration experiment with one of thesubscriber units, the method enables subscriber dependent uplink anddownlink signatures to be determined for any subscriber unit, thesesignatures including the effects of the electronic signal paths in thebase station hardware and any differences between the uplink anddownlink electronic signal paths for the subscriber unit. One use ofsuch information is to determine separate calibrations for eachsubscriber unit when the RF propagation to and from the subscriber unitis different. Another use is for calibrating the base station, butrather than obtaining a single calibration vector using the base stationand a single subscriber unit, using several subscriber units todetermine the single calibration vector. In one embodiment, the singlecalibration vector is the average calibration vector. In anotherembodiment, it is the weighted average calibration vector, the weightinggiven to the estimate made using a particular subscriber unit dependenton a measure of the quality of the signal received by that subscriberunit, so that estimates from subscriber units having better qualitysignals are weighed more in the weighted average. A method and apparatusfor determining signal quality is disclosed in above referenced U.S.patent application Ser. No. 09/020,049. The implementation of the signalquality estimation method is now described.

[0089] Denote by N the number of samples of a burst to use for theestimate. The sampled modulus information is first extracted by formingthe sum of the squares of the in phase and quadrature received signals.The mean power and mean squared power are then determined using averagesover the number of samples for the expectation operation.${\overset{\_}{R^{2}} = {{\frac{1}{N}{\sum\limits_{t = 1}^{N}{I^{2}(t)}}} + {Q^{2}(t)}}},\text{and}$$\overset{\_}{R^{4}} = {\frac{1}{N}{\sum\limits_{t = 1}^{N}{\left( {{I^{2}(t)} + {Q^{2}(t)}} \right)^{2}.}}}$

[0090] Note that once the instantaneous power R²(t)=I²(t)+Q²(t) isdetermined, determining the squared power R⁴(t)=[R²(t)]² requires only asingle additional multiplication per sample, and the estimatedsignal-to-interference-plus-noise-ratio (SINR) is determined as thesignal quality estimate, preferably with at most one square rootoperation, using $\begin{matrix}{{SINR} = \quad \frac{\sqrt{2 - \frac{\overset{\_}{R^{4}}}{\left( \overset{\_}{R^{2}} \right)^{2}}}}{1 - \sqrt{2 - \frac{\overset{\_}{R^{4}}}{\left( \overset{\_}{R^{2}} \right)^{2}}}}} \\{{= \quad \frac{A - \sqrt{A}}{1 - A}},{{\text{where}\quad A} = {2 - \frac{\overset{\_}{R^{4}}}{\left( \overset{\_}{R^{2}} \right)^{2}}}}}\end{matrix}$

[0091] Both the ratio {overscore (R⁴)}/({overscore (R²)})² and thequantity A are sometimes called the kurtosis.

[0092] This preferred method of signal quality estimation is insensitiveto frequency offset, and so is a particularly attractive method for usewith the CM method which also is insensitive to frequency offsets.

[0093] In alternate embodiments, the single calibration vector estimatemay be obtained using some other function of the several determinationsof calibration vectors, for example, taking from each calibration vectoronly the good quality estimates of the element, and then combining allthe subsets to obtain one high quality calibration vector.

[0094] Note that in the description below, the phase and magnitudedistortions that occur in the various signal paths are described by theamplitude and phase, respectively, of a single complex valued number, sothat a calibration for a one-to-M or M-to-one system is described by aM-dimensional complex valued vector. For a FDMA or FDMA/TDMA system, adifferent complex number may be required to describe the phase andmagnitude distortions for each carrier (each frequency band).

[0095] Also note that often, while the electronics may be adequatelydescribed by a simple phase and amplitude factor, the RF propagationpart within each frequency band of a carrier is not adequately describedby a complex number, but is adequately described by a transfer function.Even in such a situation, with reciprocity in the RF paths between theuplink and downlink, the transfer functions cancel out when used forcalibration, so that a complex number adequately describes thecalibration for one antenna's uplink-downlink signal path, and a complexvalued M-dimensional calibration vector is adequate.

[0096] Sometimes, even the signal paths through the receiver electronicsor transmit electronics or both are not adequately describable bycomplex numbers, but are describable by transfer functions. In analternate implementation, this is taken into account, so each of theuplink and each of the downlink signal paths is described by a complexvalued transfer function for a baseband signal. How to extend theimplementations described herein to take into account a set offrequencies rather than a frequency-independent (within a carrier band)phase and amplitude baseband signal path description would be clear toone of ordinary skill in the art, and the scope of this inventioncertainly includes such extension.

[0097]FIG. 2 shows how the uplink and downlink channel descriptions arefurther mathematically decomposed into the product of “propagation” and“electronic” factors in the following manner. Between each base stationantenna element (an element in 105) and the antenna 205 of thesubscriber unit, there is a complex valued number that describes thephase and amplitude distortion that occurs in a baseband signal due tothe RF propagation effects on the uplink and on the downlink. Suchpropagation effects include without limitation path loss, fading andshadowing effects, multipath, and near-field scattering. For each of theuplink and the downlink, the M such numbers can be combined asM-dimensional complex valued vectors. Define g_(rx) and g_(tx) as thesevectors for the uplink and downlink, respectively. g_(rx) and g_(tx) arecalled the propagation factors herein. In a typical low-mobilityenvironment the propagation factors remain constant over several frames(i.e., tens to hundreds of milliseconds).

[0098] Similarly, there is a complex valued number that describes thephase and amplitude distortion that occurs in a baseband signal due tothe receive electronics between an element of the antenna array 105 andthe corresponding output terminal of receive signal processor 111, andanother complex valued number that describes the phase and amplitudedistortion that occurs in a baseband signal in the transmit electronicschain between an input terminal of transmit signal processor 123 and thecorresponding element of the antenna array 105. These electronics chainphase and amplitude distortions include those that occur due to cablelosses, imperfect physical connections, variations in the gains of thevarious active receive or transmit RF electronics, and group delays inthe particular components that are included in the RF electronics, forexample surface acoustic wave (SAW) filters and other components. If thebase-station hardware is stable, the electronic factors remain constantover an extended period of time (minutes, hours or days). There are Melectronics based factors for each of the transmit and receiveelectronics chains. For each direction, these factors can be combined asan M-dimensional complex valued vector. Define receive electronic factorvector e_(rx) as the vector of distortions of the M receive electronicschains, and transmit electronics factor vector e_(tx) as the set ofdistortions for the M transmit electronics chains.

[0099] In FIG. 2 the uplink propagation factors vector g_(rx) is shownas 211 and the uplink electronic factors vector e_(rx) is shown as 215,while the downlink electronic factors vector e_(tx) is shown as 217 andthe downlink propagation factors vector g_(tx) is shown as 219.

[0100] The multiplicative nature of these factors for each antennaelement for each direction may be mathematically expressed as

a _(rx) =g _(rx) {circle over (x)}e _(rx)

a _(tx) =g _(tx) {circle over (x)}e _(tx)  (1)

[0101] where {circle over (x)} denotes the element wise product (i.e.,the Hadamard product).

[0102] The preferred embodiment system is a frequency division multipleaccess/time division multiple access (FDMA/TDMA) system in which eachconventional channel is a time slot in a frequency channel (a frequencychannel is referred to as a “carrier” herein for FDMA/TDMA systems). Inparticular, time is divided into frames of timeslots and such a frame isshown as 301 in FIG. 3. Frame 301 of the preferred embodiment includeseight timeslots. In order, there are four receive timeslots labeled 0through 3 (items 305, 307, 309, and 311) followed by four transmittimeslots labeled 0 through 3 (items 315, 317, 319, and 321) in FIG. 3.Thus, in the preferred embodiment, the uplink and downlink factors aremeasured over consecutive receive and transmit slots that are separatedby a relatively short time interval. Therefore, by the principle ofreciprocity, it is reasonable to assume that the uplink and downlinkpropagation factors are identical:

g _(rx) =g _(tx)  (2)

[0103] In an FDD system the relation between the uplink and downlinkpropagation factors may be more complicated, and can still bedetermined.

[0104] Uplink Weight Computation

[0105] In the preferred embodiment, the uplink weights are computed atbase station 101 by receive signal processor 111. The uplink weights aresummarized by a complex valued M-dimensional complex valued receiveweight vector (also called uplink weight vector) 115 denoted by w_(rx)herein, each element of which describes the weighing in amplitude andphase of the baseband received signals. The result of applying theweighting generates a baseband signal from the particular subscriberunit. Referring to FIG. 1, the received signals 106 from the antennaelements are digitized and converted to baseband by receive RFelectronics unit 109. FIG. 4 shows the preferred embodiment (byprogramming) of receive signal processing unit 111, including receive(uplink) weights computation. Receive signal processor 111 firstperforms pass-band filtering, and compensates for frequency offset,timing offset, I/Q mismatch, and other possible distortions. Theseoperations are commonly labeled as “preprocessing,” and are carried outin the preprocessor shown as 403 in FIG. 4.

[0106] In the next step the transmitted symbol sequence 411 is estimatedfrom the set of preprocessed received signals 405 by using a suitablespatial processing and demodulation technique. Referring to FIG. 4, anestimate of the signal from the particular desired subscriber unit isdetermined by spatial processor 407 by weighting in amplitude and phaseby a set of receive weights described by a receive (uplink) weightvector 115.

[0107] Note that the invention also covers replacing spatial processor407 with a spatio-temporal processor which includes time equalization.With spatio-temporal processing, the weighting is replaced by aconvolution operation in the time domain, or equivalently,multiplication in the frequency domain. The convolution usually isfinite and on sampled data, and so is equivalent to combining thespatial processing with time equalization using a time-domain equalizerwith a finite number of equalizer taps. That is, each of the weights inthe weight vector is replaced by a finite number of values. If thelength of each convolving function is K, then rather than determining acomplex valued M-weight vector w_(rx), one determines a complex valued Mby K matrix W_(rx).

[0108] Note that a spatial weight determining method can easily bemodified for spatio-temporal processing according to a weight matrix byre-expressing the problem in terms of matrices and vectors of differentsizes. As throughout this description, let M be the number of antennaelements, and N the number of samples. Let K be the number of timeequalizer taps per antenna element. A set of received signal samples canbe written as a matrix of row vectors, each row vector representing thesingle samples from a single antenna. All the signal samples can then berepresented by an (M by N) received signal matrix. To accommodatespatio-temporal processing, each row vector of N samples of the (M by N)received signal matrix can be rewritten as K rows of shifted versions ofthe first row to produce a received signal matrix of size (MK by N),which when pre-multiplied by the Hermitian transpose (i.e., complexconjugate transpose) of a weight vector of size (MK by 1) produces anestimated received signal row vector of N samples. The spatio-temporalproblem has thus been re-expressed as a weight vector determiningproblem. For example, for covariance based methods, the weight vector isa “long” weight vector of size (MK by 1). Rearranging terms in the“long” weight vector provides the required (M by K) weight matrix.Therefore, while the description herein is in terms of weights andspatial processing, the scope is intended to include spatio-temporalprocessing.

[0109] Referring again to FIG. 4 and processor 407, at first, anestimate of the uplink weight vector 115 is used, for example the valuefrom the previous frame. The signal estimate 408 is then demodulated bydemodulator and reference signal generator 411 to generate the estimateof the transmitted symbol sequence 412 which then is further processedby higher level processing unit 413 to generate the voice or data signal113 that is sent to the Network Interface Unit (not shown). In additionto producing the symbol sequence 412, demodulation and reference signalgenerator 411 also produces a reference signal 410 which is a modulatedsignal that is modulated by the estimated symbols and that has a correctsignal structure according to the particular modulation protocol used.This reference signal, together with the preprocessed receive signal set405, is used by weight vector generator 409 to generate a betterestimate of the receive weight vector 115. Weight vector generator 409implements an optimization method that determines the weight vector thatminimizes an objective function of weight vectors, the objectivefunction including a measure of the deviation of the signal generatedthrough a signal copy spatial processing operation using the weightvector to the reference signal 410. In the preferred embodiment, theobjective function also includes a term to limit the magnitude of theweight vector. The next estimate of the weight vector obtained fromweight vector generator 409 can then be used by signal copy operation407 and also may be used by transmit weight generator 117. For moredetails on the structure of the base station on which the method of thepresent invention is preferably implemented, see above referenced U.S.patent application Ser. No. 09/020,049. For further details of theuplink weight vector computation, see above-referenced U.S. patentapplication Ser. No. 08/729,390 and U.S. patent application Ser. No.09/153,110 for METHOD FOR REFERENCE SIGNAL GENERATION IN THE PRESENCE OFFREQUENCY OFFSETS IN A COMMUNICATIONS STATION WITH SPATIAL PROCESSING.

[0110] Downlink Weight Computation

[0111] The downlink weights 118 may be expressed as an M-dimensionalcomplex valued vector of weights w_(tx) (called the transmit weightvector, also the downlink weight vector). In the preferred embodiment,the downlink weights are computed directly from the uplink weights. Thesymmetry of the uplink and downlink signal paths is used. This symmetry,illustrated in FIGS. 5A (uplink) and 5B (downlink), may be expressed asfollows:

[0112] 1. The impulse response of the scalar “channel” (in baseband)between the modulated baseband signal (shown as 503) transmitted by thesubscriber unit and the post-spatial processing (i.e., demultiplexed)signal (for example, referring to FIG. 4, the reference signal 410) issubstantially the same as the opposite direction impulse response fromthe pre-spatial processing scalar baseband signal 507 transmitted fromthe base station to the received baseband signal 509 at the subscriberunit. Mathematically, this symmetry may be stated as the uplink anddownlink weight vectors substantially satisfying the equation

w* _(rx) a _(rx) =w* _(tx) a _(tx).  (3)

[0113] 2. For receiving from and transmitting to the same subscriberunit (assuming the subscriber unit uses the same antenna for receive andtransmit), the beam pattern of the antenna array on the uplink and thedownlink should be substantially identical. In the case that thereciprocity condition (g_(rx)=g_(tx)) substantially holds, this meansthat the weight vectors should substantially satisfy

w _(rx) {circle over (x)}e _(rx) =w _(tx) {circle over (x)}e _(tx),  (4)

[0114] where {circle over (x)} denotes the elementwise product (i.e.,the Hadamard product). Note that in general the beam pattern of theantenna array depends on the weight vectors, as well as on the transferfunctions of the RF electronics.

[0115] Eq. (3) has many solutions for w_(tx) while Eq. (4) has only onesolution:

w _(tx) =w _(rx) {circle over (x)}e _(rx) Øe _(tx),  (5)

[0116] where Ø denotes elementwise division. Consequently, the mainequation that governs the transmit weight generation is given by

w _(tx) =w _(rx) {circle over (x)}c,  (6)

[0117] where the calibration vector 133 (denoted by c) is defined as

c=e _(rx) Øe _(tx).  (7)

[0118] The internal structure of the transmit weight generator 117 isdepicted in FIG. 6. To generate an element of transmit weight vector118, the corresponding element of calibration vector 133 is multipliedby the corresponding element of the receive weight vector 115 usingelementwise multiplication process 603.

[0119] The Calibration Process

[0120] The main purpose of the calibration process is to determinecalibration vector 133 for a base station and one of its subscriberunits which supports the calibration procedure. No additionalcalibration equipment such as a transponder, signal generator, ormeasuring network is needed. In a typical TDD system the calibrationprocess consists of the following steps:

[0121] 1. Establish a connection with a suitable subscriber unit;

[0122] 2. Estimate the uplink channel spatial signature a_(rx);

[0123] 3. Estimate the downlink channel spatial signature a_(tx);

[0124] 4. Assuming reciprocity, compute the calibration vector 113 as

c=a _(rx) Øa _(tx) =e _(rx) Øe _(tx);  (8)

[0125] 5. Terminate the connection with the subscriber unit.

[0126] Clearly in order to determine calibration functions, one need notexplicitly display or store uplink and downlink signatures (steps 2 and3 above) and one may instead proceed directly to step 4 of computing thecalibration function from intermediate quantities related to the uplinkand downlink signatures. For the purposes of this invention thecomputation of the calibration function from such intermediatequantities is equivalent to computing the calibration function fromuplink and downlink signatures.

[0127] In the current WLL system in which the preferred embodiment isimplemented, each subscriber unit is able to support the calibrationmethod. Nevertheless, to maximize the signal to noise ratio, it isgenerally recommended to choose a subscriber unit that is close to thebase station. Calibration calls can be initiated on any carrier and anytime slot while the base station is servicing standard traffic channel(TCH) calls on other carriers and time slots.

[0128] Note that while the description herein is for the calibration tooccur by the base station communicating with a subscriber unit, thescope clearly includes the base station communicating with a specialpurpose transceiver that performs the functions described herein, whilenot necessarily performing any other functions, for example the typicalfunctions a typical subscriber unit performs. For example, one can use asubset of the hardware and software included in a subscriber unit tocarry out the calibration.

[0129] Note that the preferred embodiment uses a system in whichcommunication occurs burst-by-burst. Hence, the description herein usesthe term “burst” and used terms such as traffic bursts, calibrationbursts, etc. The invention certainly is not limited to burst-by-burstsystems. The general equivalent term to “burst” applicable to bothburst-by-burst and non burst-by-burst systems used herein is “waveform”,and therefore a “calibration waveforms” is a calibration burst for abusts-by-burst system, a “traffic waveforms” is a traffic (or TCH) burstfor a busts-by-burst system, and so forth.

[0130]FIG. 7 shows a typical protocol which includes a calibration callaccording to aspects of this invention. Different protocols may bedesigned for other implementations. The sequence order is from top tobottom. The direction of arrows shows the direction of communication.The protocol starts with a standard call-setup procedure 703 whichincludes a paging call 711 from the base station to the subscriber unit,a link channel request 713 from the subscriber unit to the base station,resulting in link channel assignment sent to the subscriber in step 715.Synchronization (“SYNCH”) bursts are then sent on the uplink (717) thenon the downlink (719). Finally, in step 721, the page response is sentto the base station. For the calibration burst phase 705 of theprotocol, the subscriber unit transmits a first uplink calibration burstor bursts (723) so that the base station can estimate the uplinkchannel. Immediately after this, in step 725, the base station transmitsa first downlink calibration burst (or bursts) so that the subscriberunit can estimate the downlink channel.

[0131] Note that in the preferred embodiment, the calibration bursts arecalibration waveforms that conform to the particular air interfacestandard, in this case, the PHS standard. By “conforming to an airinterface standard” we mean conforming to the channel structure andmodulation of an air interface, where “channel structure” is a frequencyslot in the case of FDMA, a time and frequency slot in the case of TDMA,or a code channel in the case of CDMA, and “modulation” is, for example,π/4-DQPSK in the case of PHS, or GMSK in the case of GSM, and so forth.In the two-tone and multi-tone calibration methods described hereinunder, the calibration waveform consists of a sum of two or morewaveforms each conforming to the PHS air interface standard. As suchsums occur naturally in a multiuser communication system with frequencyreuse, a sum of waveforms conforming to an air interface standard isalso considered to conform to an air interface standard for the purposeof this description.

[0132] While one implementation would be to calibrate the whole antennaarray at once, in the preferred embodiment, one considers not the wholearray of M antenna elements, but subarrays of the array, each of lessthan M elements, and calibrates each subarray independently. In thispreferred implementation, one or more additional uplink calibrationbursts and one or more additional downlink calibration bursts mayneeded, each for each additional subarray, and these additional stepsare shown as dotted lines 727 and 729, respectively in FIG. 7. Note thatwhile only one downlink and one uplink additional step is shown dotted,it is to be understood that this represents as many additional bursts asthere are additional subarrays to be calibrated.

[0133] In the particular implementation, the antennas are calibratedpairwise with each antenna calibrated with respect to a fixed referenceantenna. Thus, the M-element antenna array is viewed as a collection of2-element subarrays and there are M-1 bursts used to calibrate in eachdirection (steps 727 and 729 each carried out M-2 times). FIG. 8 shows acircular arrangement of 6 antennas 801, 802, 803, 805, 807, and 809,with antenna 801 arbitrarily chosen as the fixed reference antenna. Thesubarrays are shown as the antennas within the dotted line areas. Thefive subarrays are: subarray #1 (811) of antennas 801 and 802, subarray#2 (813) of antennas 801 and 803, subarray #3 (815) of antennas 801 and805, subarray #4 (817) of antennas 801 and 807, and subarray #5 (819) ofantennas 801 and 809.

[0134] In the preferred embodiment, the subscriber unit has someintelligent signal processing capabilities which allow it to analyze thedownlink calibration burst or bursts. In general, some of the downlinkchannel estimation can then be carried out by the remote subscriberunit, this part of the signature estimation determining partial results,called “downlink signature related signals” herein. In the preferredembodiment, the subscriber unit has sufficient processing power tocompletely compute the downlink channel estimate, and in this case, thedownlink signature related signals are the downlink channel estimatecomponents. These results (whether complete or partial estimates—ingeneral, downlink signature related signals) are sent back to the basestation by using some standard messaging protocol, including withoutlimitation SACCH, FACCH, TCH payload as described in the PHS protocol.The PHS protocols are incorporated herein by reference. The PHS standardis described, for example, in the Association of Radio Industries andBusinesses (ARIB, Japan) Preliminary Standard, Version 2, RCR STD-28 andvariations are described in Technical Standards of the PHS Memorandum ofUnderstanding Group (PHS MoU—see http://www.phsmou.or.jp). This sendingis shown as step 731 for the first downlink calibration burst and asdotted line 733 for those implementations that use additional bursts,for example for the remaining subarrays. Other relevant information(e.g., signal quality estimates or the raw I/Q samples) can also betransmitted back to the base station from the subscriber unit for use inpower control and for other analyses and purposes. See above referencedU.S. patent application Ser. No. 09/020,049 for a description of thepower control and signal quality estimation aspects of a subscriberunit.

[0135] At the end of the calibration process, the base station computesthe calibration vector and terminates the calibration call. The calltermination 709 preferably includes a disconnect command 735 from thebase station followed by a release message 737 from the subscriber unit.

[0136] Uplink Signature Estimation

[0137] In the preferred embodiment, uplink signature estimation occursat an active subscriber unit in the vicinity of the base station. Afterthe service channel is established, the subscriber unit transmits anuplink calibration burst towards the base station. In our particularimplementation, the uplink calibration bursts are idle (no-payload) TCHbursts. In alternate embodiments, other sequences can be used, and howto modify the method to use other sequences would be clear to one ofordinary skill in the art. For example, in another embodiment, downlinksignature estimation is carried out first. The downlink signaturerelated signals computed at the subscriber unit, which preferably arethe signature estimates, are then transmitted to the base station. Thesesignals are then used to estimate the uplink signature.

[0138]FIG. 9 describes the elements for determining the uplink signaturea_(rx). In the preferred embodiment, subscriber unit (e.g., unit 141)includes an uplink calibration burst synthesizer 907 implemented as aset of programming instructions on a signal processor. Synthesizer 907includes a memory (part of the already present signal processor memory),and generates the first calibration burst (in step 723) or the secondcalibration burst (in step 727). The burst is transmitted from thesubscriber unit antenna 911 using the subscriber unit's transmit RFelectronics 909. The architecture of the preferred embodiment subscriberunit is described in above referenced U.S. patent application Ser. No.08/907,594 and in FIG. 12. Referring to FIG. 12, time duplexer 1203 isin the transmit position during transmission and connects the output oftransmit RF electronics 909 to antenna 911. Normal traffic burst signalsare obtained from telephony interface unit 1213 via a vocoder DSP 1209.The complex valued (I, Q) samples are formed in a DSP device (TX DSP1211) which is connected to a memory 1207 shared with another DSPdevice, the RX DSP 1205 used for signal reception. For the uplinkchannel determination implementation described herein, TX DSP 1211 isprogrammed to carry out the function of uplink calibration burstsynthesizer 907 in addition to its normal transmit signal processingfunctions. The uplink calibration bursts are received by the basestation antenna array 105 and converted to the baseband signals 110 bythe receive RF electronics 109, as shown in FIG. 9. The signals from theantenna elements are then processed by the receive signal processor 111which is made up of one or more digital signal processing devices (DSPs)programmed to carry out the functions of the elements 403, 921, and 931.Pre-processor 403 carries out pre-processing which includes basebandfiltering, and removing the frequency offset, the timing offset, and theI/Q mismatch from the received signal. In some implementations, basebandequalization may also be included in the pre-processor 403 if necessary,and how to so include equalization and would be clear to those skilledin the art and is not the main concern of the invention. Unit 921includes units 407 and 411 and estimates the transmitted symbol sequence(a reference signal) by carrying out the signal copy operation,demodulation and reference signal generation. In the preferredembodiment, the subscriber unit transmits standard TCH bursts, andtherefore the default TCH demodulation method of the base station can beused for this purpose. In an alternate embodiment, the subscriber unittransmits a pre-defined calibration sequence that is explicitly known,and thus may be pre-stored at the base station. In this case, it is notnecessary to demodulate the received signal. This alternate is shown indotted lines in FIG. 9, where the pre-defined burst segments 923 areused instead of the transmitted signal estimate 410. Channelidentification unit 931 uses the transmitted signal estimate 410 andreceived signals 405, which are the input and output signalsrespectively, of the uplink channel, 933 to estimate the underlyingspatial signature 933. Any standard system identification technique maybe used in channel identification unit 931. The following method is usedin the preferred embodiment. N samples of the received signals 405 andthe transmitted signal estimate 410 are used. In the preferredembodiment, N=50. That is, just 50 samples of the burst are used. Denoteby k the time index of the N samples, where k=0,1, . . . , N−1, by x(k)vector of received signals 405 at time k, and by s(k) transmitted signalestimate 410 at time k. The estimate of the uplink channel signature isobtained as

â _(rx) =XS*(SS*)⁻¹  (9)

[0139] where matrix X=[x(0)x(1) . . . x(N−1)] and vector S=[s(0)s (1) .. . s(N−1)]. Those skilled in the art may recognize this as the maximumlikelihood estimate of the channel signature for modeling the receivedsignals by

x(k)=a _(rx) s(k)+v(k), k=0,1, . . . , N−1  (10)

[0140] where v(k) denotes a vector of additive noise at time k, thenoise vector being a vector of statistically independent, identicallydistributed Gaussian random processes with a mean E{v(k)}=0 andcovariance matrix E{v(k)v(k)*}=σ_(v) ²I, where I is the identity matrix.This part of the invention however does not depend on any modelingassumptions. In alternate embodiments, more or less sophisticatedstandard system identification techniques may be used in place of Eq.(9). The book by Lyung, L., System Identification: Theory for the User,Englewood-Cliffs, N.J.: Prentice-Hall, 1987 is a good source for manyalternate system identification methods that may be adapted for use inthe present invention. Note also that the solution of Eq. (9) andequivalent solutions are sometimes referred to herein as the maximumlikelihood estimates, even when the received signal model and otherconditions for the maximum likelihood are not met, and it is to beunderstood that the term “maximum likelihood estimate” means thesolution that would be maximum likelihood when the appropriate linearsignal model and noise conditions hold. For example, applying Eq. (11)or equivalent would fall under “maximum likelihood estimate” for anytransmitted S and received X using any or no model with any kind ofnoise present.

[0141] Downlink Signature Estimation

[0142] In order to estimate the downlink channel, the base station 101transmits one or more downlink calibration bursts towards subscriberunit 141. FIG. 10 describes the elements for determining the downlinksignature a_(tx). In the preferred embodiment, transmit signal processor123 in base station 101 is programmed as a downlink calibration burstsynthesizer 1005 to generate the downlink calibration burst (the firstburst of step 725 or the second burst of step 727 depending on thenumber of bursts used in the embodiment of the method, and the step inthat embodiment). Such a burst preferably is generated by recalling theburst from a memory in base station 101. The bursts are transmitted tosubscriber unit 141 by using transmit signal processor 123 for therequired spatial processing (shown in FIG. 10 as part of unit 1005) andthen transmitting through the transmit RF electronics 125 and antennaarray 105.

[0143] The bursts are received in the subscriber unit (e.g., unit 141)on antenna 911 via subscriber unit receive electronics 1009. Referringagain to FIG. 12, the preferred embodiment subscriber unit includes RXDSP 1205 which for this implementation is programmed as a pre-processor1011 to generate a sampled received signal 1012 denoted y(k) where k isused as a time index, and also programmed as a downlink channelidentification processor 1013 which determines the downlink channelsignature using the received signal 1012 and a stored version 1019 ofthe set of transmitted signals denoted by M-vector z(k). The storedversion 1019 is stored in a buffer formed in memory 1207. The subscriberunit then transmits the result back to the base station.

[0144] In the particular embodiment, the signals are modulated usingπ/4DQPSK and have a baud rate of 192 kbaud per sec. The received signaly(k) is four times oversampled. When used for two-tone calibration (seebelow), the transmitted calibration waveforms are appropriatelymodulated sine waves, and in the preferred embodiment, to preservememory, only a single period of each sine wave is stored in memory 1207,that section of memory 1207 configured as a circular buffer. The datathen is repeatedly read out as a sequence of periods.

[0145] A typical subscriber unit usually has at most a few antennas (oneantenna 911 in the WLL system on which the invention preferably isimplemented), which limits the information that is available fordownlink signature estimation. Also, the hardware for a typicalsubscriber unit is simple because of size and cost constraints andtherefore less capable of sophisticated, accurate processing than atypical base station's hardware. As a result, the received signal at thesubscriber unit may have significant distortions including, withoutlimitation, frequency and timing offset effects, and phase noise thatmay reduce the accuracy of the downlink channel estimate compared, forexample to those of the uplink estimate. In the future, it isanticipated that more signal processing (or other computing) power willbe available in average subscriber units to enable these distortions tobe corrected in preprocessor 1011. However, our invention also workswhen less signal processing power is available.

[0146] In an improved embodiment, the base station uses specificallydesigned signal sequences that are robust with respect to effects thatinclude, without limitation, frequency offset, timing offset, I/Qmismatch, and phase noise. This enables accurate results to be obtainedusing even simple inexpensive subscriber units with some, but limited,signal processing capability. For example, the downlink calibrationburst may consist of pure tones. This enables RX DSP 1205 programmed aspreprocessor 1011 in the subscriber unit to carry out frequency offsetand timing alignment estimation with little computational effort.Alternatively, the downlink calibration burst can be synthesized frompseudo-random signal sequences or chirp (swept frequency) signalsequences which make it possible to characterize the propagation channelacross a wider range of frequencies.

[0147] Let row vector z(k)=[z₁(k) z₂(k) . . . z_(M)(k)], k=0, 1, . . . ,N−1 denote the N samples (in baseband) of M modulated baseband signalsz₁(k), z₂(k), . . . , z_(M)(k) that are transmitted from base station101 from a calibration burst. Let y(k) k=0, 1, . . . , N−1 denote the Nsamples of the received signal (in baseband and after the preprocessingof 1011) at the subscriber unit. Define vector y and matrix Z as${y = {{\begin{bmatrix}{y(0)} \\{y(1)} \\\vdots \\{y\left( {N - 1} \right)}\end{bmatrix}\quad Z} = \begin{bmatrix}{z_{1}(0)} & {z_{2}(0)} & \cdots & {z_{M}(0)} \\{z_{1}(1)} & {z_{2}(1)} & \cdots & {z_{M}(1)} \\\quad & \quad & \vdots & \quad \\{z_{1}\left( {N - 1} \right)} & {z_{2}\left( {N - 1} \right)} & \cdots & {z_{M}\left( {N - 1} \right)}\end{bmatrix}}},$

[0148] respectively. The downlink signature estimate 1017 is preferablydetermined in identification processor 1013 according to

â _(tx)=(Z*Z)⁻¹ Z*Y.  (11)

[0149] Those skilled in the art may recognize that this is the maximumlikelihood estimate of the downlink signature when the received signalsamples 1012 conform to the model (in baseband) that

y(k)=z(k)a _(tx) +n(k), k=0,1, . . . , N−1  (12)

[0150] where the n(k), k=0, . . . , N−1 denote some additive noise inthe received signal, modeled as N statistically independent, identicallydistributed Gaussian random variables. Note that this invention does notdepend on the received signal samples conforming to such a model. Notealso that the solution of Eq. (11) and equivalent solutions aresometimes referred to herein as the maximum likelihood estimates, evenwhen the received signal model and other conditions for the maximumlikelihood are not met, and it is to be understood that the term“maximum likelihood estimate” means the solution that would be themaximum likelihood solution when the appropriate linear signal model andnoise conditions hold. For example, applying Eq. (11) or equivalentwould fall under the term “maximum likelihood estimate” for anytransmitted Z and received Y using any or no model with any kind ofnoise present.

[0151] Denoting the noise samples as a vector ${n = \begin{bmatrix}{n(0)} \\{n(1)} \\\vdots \\{n\left( {N - 1} \right)}\end{bmatrix}},$

[0152] Eq. (12) can then be expressed as

y=Za _(tx) +n.  (13)

[0153] Note that the signature 1017 can be determined according to Eq.(11) only if Z has linearly independent columns. For this, each antennaelement of the calibrated array (or subarray) transmits M (or fewer inthe case of a subarray) substantially “linearly independent” signalsfrom the M (or fewer) antenna elements during downlink calibration. Mtransmitted signals z_(i)(k) are linearly independent if it isimpossible to find constant complex valued parameters c₁, c₂, . . . ,c_(M) so that Σ₁₌₁ ^(M)c_(i)z_(i)(k)=0 for k=0, 2, . . . , N−1 . Inpractice, this requirement can be fulfilled in various different ways.In one embodiment, the calibration burst can be divided into segments sothat only one antenna element is active at any given time (orthogonalityin the time domain), Alternatively, the antenna elements can transmitpure tones with different frequencies (orthogonality in the frequencydomain). Linearly independent signals can also be synthesized frompseudo-random signal sequences or chirp signal sequences. Othertechniques would be apparent to those of ordinary skill in the art.

[0154] Two-Tone Downlink Calibration

[0155] In the preferred embodiment, the antenna array is partitionedinto 2-element subarrays with a common reference element, as shown inFIG. 8, and each subarray is calibrated independently. In oneembodiment, during calibration each antenna element of a particularsubarray transmits a complex valued sine wave at a different frequency.Denote by (ω₁ and ω₂ (in radians per second) the frequencies of thefirst calibration signal through the first antenna element of aparticular subarray and the second calibration signal through the secondantenna element of a particular subarray, respectively. In this case,the value of M is 2 and the downlink channel estimate according to Eq.(11) is $\begin{matrix}{\begin{bmatrix}{\hat{a}}_{1} \\{\hat{a}}_{2}\end{bmatrix} = {\begin{bmatrix}N & \frac{^{{j\Delta\omega}\quad N\quad T} - 1}{^{{j\Delta\omega}\quad T} - 1} \\\frac{^{{- {j\Delta\omega}}\quad N\quad T} - 1}{^{{- {j\Delta\omega}}\quad T} - 1} & N\end{bmatrix}^{- 1}\begin{bmatrix}{\sum\limits_{k = 0}^{N - 1}{{y(k)}^{{- {j\omega}_{1}}k\quad T}}} \\{\sum\limits_{k = 0}^{N - 1}{{y(k)}^{{- {j\omega}_{2}}k\quad T}}}\end{bmatrix}}} & (14)\end{matrix}$

[0156] where T denotes the sampling period for the signals and Δω=ω₂−ω₁denotes the frequency separation between the tones. If N is chosen sothat the observation interval NT is an integer multiple of 2π/Δω, thene^(jΔωNT)=1, and we obtain the simple formulas $\begin{matrix}{{{\hat{a}}_{1} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{{y(k)}^{{- {j\omega}_{1}}k\quad T}}}}},} & \text{(15a)} \\{{\hat{a}}_{2} = {\frac{1}{N}{\sum\limits_{k = 0}^{N - 1}{{y(k)}{^{{- {j\omega}_{2}}k\quad T}.}}}}} & \text{(15b)}\end{matrix}$

[0157] One will recognize these as the discrete Fourier transform (DFTor its rapid implementation, the FFT) of the received signal at ω₁ andω₂ respectively. One also will recognize these as proportional to thecross-correlations of the received subscriber unit signal y with the twocalibration bursts, respectively. Clearly, in implementation, the 1/Nfactors are not included in determining the signatures.

[0158] The relative downlink signature for one of the antenna elements,say the second antenna element, with the first antenna element as thereference, is computed as the second cross correlation divided by thefirst cross correlation.

[0159] In the preferred embodiment implementation, RX DSP 1205 isprogrammed as downlink channel identification processor 1013. Receivedsignal samples y(k) are four times oversampled 192 kbaud per sec.signals. That is, there are 784 ksamples per second. The two frequenciesused are 24 kHz (divided by 2π for kradians/sec.) and −72 kHz (recallthat the calibration signals are complex valued). In general, the largerthe frequency difference Δω=ω₂−ω₁, the better the performance. In thepreferred implementation, signals are synthesized by providingparticular bit patterns to the π/4 DQPSK modulator (the standard forPHS). This enables the tones to be easily synthesized. However, the π/4DQPSK modulation and the particular baud rate means that effectivelyonly signals with frequencies of +72 kHz, +24 kHz, −24 kHz and −72 kHzmay be synthesized. While the greatest separation would be obtained withthe tone pair being at +72 kHz and −72 kHz, the 72 kHz signals appearless like pure tones than the 24 kHz signals, so the two tones used inthe preferred embodiment are +24 kHz and −72 kHz. That this performsbetter than using +24 kHz and −24 kHz tones is discussed in the“Performance” section herein below. The DSP program implementing channelidentification processor 1013 may be summarized as follows:

Two-Tone Downlink Procedure

[0160] INPUTS: subscriber received sequence y(0), y(1), . . . , y(N-1).

[0161] OUTPUT: The estimated downlink channel in the form$\begin{bmatrix}1 \\C\end{bmatrix}.$

[0162] 1. Cross-correlate the received sequence with the firstcalibration sequence (tone at frequencey ω₁);$A = {\sum\limits_{k = 0}^{N - 1}{{y(k)}{^{{- {j\omega}_{1}}{kT}}.}}}$

[0163] 2. Cross-correlate the received sequence with the secondcalibration sequence (tone at frequencey ω₂):$B = {\sum\limits_{k = 0}^{N - 1}{{y(k)}{^{{- {j\omega}_{2}}k\quad T}.}}}$

[0164] 3. Compute the desired quantity C=B/A.

[0165] Note that alternate implementations may use different methods forsynthesizing the tone signals that do not include the limitations ofwhat tones are available, such methods possibly requiring more compleximplementation, or may use different orthogonal signals.

[0166] The method using tone calibration bursts is robust with respectto phase noise and frequency offset for frequency offsets and phasenoises that are small compared to the frequency difference Δω.

[0167] When large timing offsets are present, an improved embodiment ofthe two-tone method allows such a timing offset to be determined and thequantities corrected for the timing offset. Let τ denote the constanttime by which the transmitted signal is delayed. In this improvedembodiment, the calibration bursts are broken up into two time segments,with the break point the same for the two bursts. During the first timesegment, a sum of the first and second sine waves is transmitted fromthe same antenna element, say the first antenna element. Let there be N₁samples during the first time segment and denote the received signal atthe subscriber unit by y₁(k), k=0, . . . , N₁−1. Assuming that the firstsegment observation interval N₁T is an integer multiple of 2π/Δω, anestimate for the timing offset is determined from the ratio of the crosscorrelation of the subscriber unit received signal with the secondcorrelation burst to the cross correlation of the subscriber unitreceived signal with the first correlation burst: $\begin{matrix}{^{{j\Delta}\quad \omega \quad \tau} = {\frac{\sum\limits_{k = 0}^{N_{1} - 1}{{y_{1}(k)}^{{- {j\omega}_{2}}k\quad T}}}{\sum\limits_{k = 0}^{N_{1} - 1}{{y_{1}(k)}^{{- {j\omega}_{1}}k\quad T}}}.}} & (16)\end{matrix}$

[0168] On the second segment of the calibration bursts, the two sinewaves are transmitted via two different antennas as in the previouslydescribed embodiment of the two-tone method. Let there be N₂ samplesduring the second time segment and denote the received signal at thesubscriber unit by y₂(k), k=0, . . . , N₂−1. If N₂ is chosen so that theobservation interval N₂T is an integer multiple of 2π/Δω, then$\begin{matrix}{\frac{\sum\limits_{k = 0}^{N_{2} - 1}{{y_{2}(k)}^{{- {j\omega}_{2}}k\quad T}}}{\sum\limits_{k = 0}^{N_{2} - 1}{{y_{2}(k)}^{{- {j\omega}_{1}}k\quad T}}} = {\frac{{\hat{a}}_{2}}{{\hat{a}}_{1}}^{{- {j\Delta}}\quad {\omega\tau}}}} & (17)\end{matrix}$

[0169] Combining Eqs. (16) and (17) leads to the desired ratio of thetwo downlink signature estimates. For simplicity, the two segments aremade of equal length, N₁=N₂. As in the first two-tone embodiment, thetwo frequencies used are 24 kHz and −72 kHz (recall that the calibrationsignals are complex valued). The DSP program for RX DSP 1205implementing channel identification processor 1013 according to thesecond implementation that includes correcting for the timing offset maybe summarized as follows.

Improved Two-Tone Downlink Procedure

[0170] INPUTS: received sequence y(0), y(1), . . . , y(N), . . . ,y(2N-1).

[0171] OUTPUT: The estimated downlink channel in the form$\begin{bmatrix}1 \\C\end{bmatrix}.$

[0172] 1. Cross-correlate the first half of the received sequence withthe first half of calibration sequence #1:${A1} = {\sum\limits_{k = 0}^{N - 1}{{y(k)}{^{{- {j\omega}_{1}}{kT}}.}}}$

[0173] 2. Cross-correlate the first half of the received sequence withthe first half of calibration sequence #2:${B1} = {\sum\limits_{k = 0}^{N - 1}{{y(k)}{^{{- {j\omega}_{2}}k\quad T}.}}}$

[0174] 3. Compute C1=B1/A1.

[0175] 4. Cross-correlate the second half of the received sequence withthe second half of calibration sequence #1:${A2} = {\sum\limits_{k = N}^{{2N} - 1}{{y(k)}{^{{- {j\omega}_{1}}k\quad T}.}}}$

[0176] 5. Cross-correlate the second half of the received sequence withthe second half of calibration sequence #2:${B2} = {\sum\limits_{k = N}^{{2N} - 1}{{y(k)}{^{{- {j\omega}_{2}}k\quad T}.}}}$

[0177] 6. Compute C2=B2/A2.

[0178] 7. Compute the desired quantity C=C2/C1.

[0179] It would be clear to those of ordinary skill in the art thatvarious modifications may be made to the methods, including withoutlimitation, using segments of unequal length, using two sets of two tonesignals (separated by a known amount), and transmitting differentcombinations. Different formulas also may be used to determine thecalibration factors.

[0180] It is advantageous to use any two constant modulus signals whosedot product is a pure tone. Alternatively, one might, for example, use atone for the first segment and a chirp signal sequence for the second.

[0181] One also may generalize the method to deal with more than twoantennas at a time. The following alternative method works for anynumber of M antennas. In the first segment (say the first half) of thesegment, the sum of M different single tone signals, each of the M tonesbeing distinct, is transmitted from the first (say the reference)antenna element, while no signal is transmitted from the other antennaelements. In the second segment, a different one of the M single tonesignals is transmitted from the M antenna elements. The method thenproceeds as follows to estimate the M-antenna element array (orsubarray). The notation used is that the first half correlations aredenoted by A_(i) with the subscript i denoting which tone the receivedsignal was correlated with, while the second half correlations aredenoted by B_(i) with the subscript i denoting which tone the receivedsignal was correlated with. The M pure tone signals have frequenciesdenoted by ω₁, ω₂, . . . , ω_(M), respectively.

Improves M-Tone Downlink Procedure

[0182] INPUTS: received sequence y(0), y(1), . . . , y(N), . . . ,y(2N-1).

[0183] OUTPUT: The estimated downlink channel in the form$\begin{bmatrix}1 \\C_{2} \\\vdots \\C_{M}\end{bmatrix}.$

[0184] 1. Cross-correlate the first half of the received sequence withthe first half of each calibration sequence to obtain M correlation A₁,A₂, . . . , A_(M), respectively.

[0185] 2. Normalize with respect to the first correlation A₁corresponding to the reference antenna element to obtain M numbers 1,A₂/A₁, . . . , A_(M)/A₁, respectively.

[0186] 3. Cross-correlate the second half of the received sequence withthe second half of each of the M calibration sequences sequence toobtain M correlations B₁, B₂, . . . , B_(M), respectively.

[0187] 4. Normalize with respect to the first correlation B₁corresponding to the reference antenna element to obtain M numbers 1,B₂/B₁, . . . , B_(M)/B₁, respectively.

[0188] 5. Compute the M signature element as 1, +* (B₂/B₁)/(A₂/A₁)], . .. , [(B_(M)/B₁)/(A_(M)/A₁)], respectively.

[0189] The above generalization for determining the signature for Melements simultaneously can be modified to avoid transmitting the sum ofall the M tones on one antenna element in the first segment. In general,one can assume that the timing offset is the same for transmissions fromall the antenna elements of a base station. In the system in which theembodiments described herein is implemented, all the ADCs and all thedownconversions and upconversions are synchronized. In such a case, forexample, only the sum of the tone transmitted from the reference antennaelement and one other antenna tone (e.g., the second) are transmittedfrom the first element in the first segment. How to modify the abovegeneralization in this and many other ways would be clear to one ofordinary skill in the art.

[0190] Note that while the above discussion mentions canceling outtiming offsets, the dividing of the factors also cancels out any phaseoffsets.

[0191] Timing Offset Determination.

[0192] The above discussion also suggests how sending multiple signals,for example, pure tone signals, can be used to determine the timingoffset in the subscriber unit with very little computation.

[0193] To determine timing offset, one carries out steps 1, 2 and 3 ofthe “Improved Two-Tone Downlink Method” above. In step 3, the quantityC1 is essentially exp−j(ω₂−ω₁)τ. Thus, taking logarithms and diving byΔω=(ω₂−ω₁) gives an estimate of the timing offset τ.

[0194] In an improved timing offset method, one carries steps 1 and 2 ofthe “Improved M-Tone Downlink Method” above. In step 2, the quantities1, A₂/A ₁, . . . , A_(M)/A₁, respectively, give the M quantities 1,exp−j(ω₂−ω₁)τ, . . . , exp−j(ω_(M)−ω₁)τ, respectively. Taking logarithmsof the last M−1 quantities and dividing the first of these by (ω₂−ω₁),the second by (ω₃−ω₁), . . . , and the last by (ω_(M)−ω₁), respectively,gives M−1 estimates of the timing offset τ. These may be averaged togive a final estimate of the timing offset

[0195] Calibration During Standard Traffic Channel Calls

[0196] In yet another alternate embodiment, instead of using dedicatedcalibration calls, it is possible to embed the calibration procedureinto standard telephone calls in both directions which are used fornormal traffic functions. Normal traffic functions depend on the airinterface, and may include demodulation, timing and frequency tracking,and various control functions such as power control and handoff. Forexample, the uplink channel signature can be estimated from standarduplink traffic channel (TCH) bursts by using a decision directedtechnique as described above. The downlink channel estimation methoddescribed above is modified as follows:

[0197] On the downlink, the base station transmits a mixture of TCHbursts and calibration bursts towards the subscriber unit in a randomfashion. That is, the calibration bursts are interspersed with the TCHbursts. Because calibration bursts may cause audible errors to occur, itis preferable to send such calibration bursts infrequently and duringsilent periods. A typical silent period is longer than a burst, so in animproved embodiment, calibration bursts are sent (instead of TCH bursts)only after a number of idle bursts are sent by the base station.

[0198] An illustrative embodiment of processing by the subscriber unitwhich includes estimating the downlink channel signature is shown inFIG. 11. In step 1105 the subscriber unit acquires the raw burst andfirst preprocesses the burst in the receive signal processor programmedas preprocessor 1011. This received preprocessed signal is stored. Thepreprocessed signal next is demodulated in step 1109 as would be astandard TCH burst. In step 1111, it is determined whether or not thedemodulated bits are for a standard TCH burst. As in, most standardprotocols, the PHS protocol used in the system of the illustrativeembodiment includes some method to determine when a sequence iscorrectly received, for example, the presence of a particularpre-defined bit-sequence. In the PHS standard, there is such a 32-bit“Unique Word” sequence, which is prearranged and known to everysubscriber unit. Correct reception is determined in step 1111 bydetecting the presence of the Unique Word. Other protocols use othertechniques, and alternate ways of determining correct reception of astandard TCH burst in whatever protocol would be clear to those ofordinary skill in the art using the specification of the protocol. Ifthe burst is determined to be a standard TCH burst, then the bitsequence is forwarded in step 1113 to vocoder DSP 1209. If, on the otherhand, the bit sequence is not recognized as a standard TCH burst, thenthe subscriber unit in step 1115 determines whether the received burstis a calibration burst. In the two-tone methods described herein above,this step 1115 is performed preferably by carrying out the firstcorrelation step of the calibration method. If the correlation is high,then there is a high level of confidence that this is a calibrationburst. If the result of step 1115 is that yes, this is a calibrationburst, then the downlink signature estimation method is continued instep 1117 and the resulting downlink signature is sent to the basestation in step 1119.

[0199] Calibration Using SYNCH Bursts

[0200] In yet another alternate embodiment, instead of using dedicatedcalibration calls, it is possible to embed the calibration bursts intoSYNCH bursts, the calibration bursts preferably being the two-segmentmulti-tone bursts (or two-segment two-tone busts for pairwisecalibration).

[0201] Performance

[0202] The accuracy of the downlink channel estimate for the two-tonemethod (improved implementation including timing alignment correction)was measured by performing experiments using the PHS base station and asubscriber unit from the WLL system used in the preferred embodiment. Inthe first experiment, two antennas of the PHS base station were usedwith two different sets of transmit electronics. Forty sets ofcalibration bursts were sent to the subscriber unit, and the subscriberunit was programmed to save the received signal. The saved receivedsignal was then used to calculate the relative downlink signature. Thecalculation was carried out offline using the MATLAB environment (TheMathworks, Inc., Natick, Mass.). The results are shown in FIG. 13. Ascan be seen, for the carrier frequency of the experiment, the twotransmit electronics/antenna elements had different amplitude gains andproduced the relative phase of about 109 degrees. The two tones usedwere +24 kHz and −72 kHz.

[0203] A second experiment was carried out, this time by using the sametransmit electronics and the same antenna. That is, the two calibrationsignals (the two tones) were transmitted from the same electronics andantenna element. FIG. 14 shows the results when the two tones used were+24 kHz and −72 kHz. As can be seen, the phase angle was close to 0.0,and the magnitude close to 1.0, as would be expected. This sameexperiment was repeated with the two tones being at +24 kHz and −24 kHz.The results are shown in FIG. 15. The error and variance when usingthese two tones were larger that when using the frequencies used forFIG. 14.

[0204] Using Several Subscriber Units

[0205] In another aspect of the invention, the calibration factor may beobtained using more than one subscriber unit and determined as afunction of signatures obtained from these subscriber units. These mayeven be all subscriber units. The function may be, for example, aprincipal component, an average, or a centroid. In the preferredembodiment of the combining step, the principal component method isused. Signatures a₁, . . . ,a_(Ns) gathered from subscribers 1, . . . ,Ns, respectively, are combined by forming a matrix A=[a₁ . . . a_(Ns)]and computing the principal component (the eigenvector corresponding theeigenvalue of largest magnitude) of A^(H)A or, equivalently, by findingthe left singular vector corresponding to the largest singular value ofA. In an improved embodiment, each subscriber unit also obtains a signalquality estimate, and these estimates are sent to the base station. Anysubscriber unit implemented signal quality determining method may beused, and the method (and apparatus) for determining signal quality usedin the preferred embodiment is the kurtosis based method disclosed inabove referenced U.S. patent application Ser. No. 09/020,049 and alsodescribed herein above. Note also that signal quality related measuresmay already be available at the base station for power control purposes.When signal quality estimates are available, a weighted averagecalibration factor is obtained, the weighting for a calibration factorusing a subscriber unit according to the received signal quality forthat subscriber unit. For example, using the principal component method,the signature estimate is the principal component of the weightedsignature matrix A=[β₁a₁ . . . β_(Ns)a_(Ns)], where β₁, . . . , β_(Ns)are the weighting factors for respective subscriber units 1, . . . , Ns.

[0206] In yet another aspect, the calibration factor may again beobtained as a function of calibration factors obtained from several(even all) subscriber units. However, the function takes into accountthe relative “quality” of each element of the signature estimate fromeach of these subscriber units. This is applicable to the case when fora subscriber unit, one or more of the base station antenna elements are“weak” compared to the other elements. In such a case, some of thesignature estimate elements and the corresponding calibration factorelements are discarded. For example, one might discard signatureelements that have a smaller (normalized) magnitude than some magnitudethreshold. Alternatively, one might use the signature estimates tocompare predicted received signals to actual received signals, and thusdetermine residual error (for example, error squared averaged over aburst) for each element and discard signature elements that produce alarge residual error. One then can combine several such “incomplete”calibration factor estimates that include at least one estimate of everyone of the calibration factor elements. As an example, suppose there arefour antenna elements in an array (or subarray), and at three subscriberunits denoted SU1, SU2, and SU3, respectively, the first and secondelements, second and third elements, and third and fourth elements,respectively, are deemed sufficiently accurate. Denoting the jthcalibration factor element using the ith subscriber unit by C_(ij), thefour elements of the complete calibration factor estimate are determinedas C₁₁, C₁₂, C₂₃(C₁₂/C₂₂), and C₃₄ (C₁₂/C₂₂) (C₂₃/C₃₃), respectively.This can be generalized to any set of complete or incomplete SUdeterminations as follows: Let C_(ij), be the jth calibration factorelement determined from the ith subscriber unit and let Q_(ij) be theestimate quality associated with the measurement of C_(ij) where i=1, .. . , Ns and j=1, . . . , M. With the above-mentioned method ofdetermining signature reliability, Q_(ij) has value 0 if the componentis deemed unreliable or value 1 if it is deemed reliable. Other methodsof mathematically indicating reliability also are possible, as will beclear to those of ordinary skill in the art. The complete calibrationvector D=[D₁D₂ . . . D_(M)] is determined by performing a jointminimization over D and the complex-valued parameters B₁, . . . ,B_(NS). That is, defining B=[B₁ . . . B_(NS)], D is determined bycarrying out the operation$\min\limits_{D}\quad {\min\limits_{B}{\sum\limits_{ij}{Q_{ij}{{{D_{j} - {C_{ij}B_{i}}}}^{2}.}}}}$

[0207] This minimization can be carried out using standard methods, forexample by performing a grid search over D to approximately locate theglobal minimum, and then performing a gradient descent to refine theestimate. Alternative methods would be clear to those of ordinary skillin the art.

[0208] Other Aspects

[0209] As will be understood by those of ordinary skill in the art, manychanges in the methods and apparatuses as described above may be madewithout departing from the spirit and scope of the invention. Variationsinclude, without limitation:

[0210] The method can be modified for estimating uplink signatures ordownlink signatures rather than only for determining a calibrationfactor to use for estimating a downlink weight vector from an uplinkweight vector.

[0211] Each uplink signature or downlink signature may be determined asa vector of transfer functions. The methods described herein would bemodified to include standard transfer function system identificationtechniques.

[0212] The uplink or downlink channel signatures may be obtained usingformulas other than derived from Eq. (9) or Eq. (11), based on differentmodels for the channels and different estimation techniques.

[0213] The uplink or downlink channel signatures may be described inother than baseband, as would be applicable to the case of the uplinkand downlink weights being applied at a base station to signals in otherthan baseband.

[0214] The methods can be adapted for different types of communicationsystems, including, without limitation, systems with mobile subscriberunits, or systems using different protocols, or both. The methods alsocan be adapted to non-digital modulated systems, such as the common AMPSFDMA system. The method also can be adapted to non TDMA digital systems.In such cases, the uplink and downlink frequencies are in generaldifferent, so that separate uplink and downlink signatures need to beobtained for each subscriber unit. Note that we can then determinedownlink weight vectors knowing all the downlink signatures for thesubscriber units.

[0215] Different pre-defined calibration signals may be used.

[0216] Different subarray configurations (of more than two antennaelements) may be used, or all the antenna elements in the arraycalibrated simultaneously.

[0217] More or less of the downlink processing can occur in thesubscriber units, depending on how much computation and storage power isavailable in the subscriber unit and the base station.

[0218] Several aspects of the invention described herein were describedimplemented as programs run on one or more DSP devices. Given sufficienteconomic incentive, DSP functionality, including DSP programs, may beincorporated into special purpose hardware, for example as part of anapplication specific integrated circuit (ASIC) or as part of a verylarge scale integrated circuit (VLSI). DSP functionality may also be metby other processors, for example a general purpose microprocessor. Inaddition, a DSP device running a program may be converted into a specialpurpose piece of hardware. Thus, the terms digital signal processor,DSP, and DSP device as used herein include these equivalentalternatives.

[0219] As will be understood by those skilled in the art, the skilledpractitioner may make many changes in the methods and apparatuses asdescribed above without departing from the spirit and scope of theinvention. For example, the communication station in which the method isimplemented may use one of many protocols. In addition, severalarchitectures of these stations and subscriber units are possible. Theinvention may be applied in a system comprising anyantenna-array-equipped transceiver and another transceiver communicatingwith the array-equipped transceiver. Many more variations are possible.The true spirit and scope of the invention should be limited only as setforth in the claims that follow.

[0220] Embodiments of the present invention may be described as:

[0221] 1. In a wireless communication system comprising a maintransceiver and a remote transceiver capable of receiving signals fromand transmitting signals to the main transceiver, the main transceivercomprising an array of transmit antenna elements, and at least onereceive antenna element, each transmit antenna element being part of atransmit electronics chain for transmitting a transmit apparatus signalusing the transmit antenna element, and each receive antenna elementbeing part of a receiver apparatus chain for receiving a receivedantenna signal from the receive antenna element, the main transceiverand the remote transceiver designed for mutual communication usingwaveforms conforming to an air interface standard, a method forestimating the downlink signature for the remote transceiver, the methodcomprising:

[0222] (a) transmitting a set of one or more downlink calibrationwaveforms from the main transceiver via the transmit antenna array tothe remote transceiver, the set of downlink calibration waveformssubstantially conforming to the air interface standard;

[0223] (b) processing the signals received at the remote transceivercorresponding to the downlink calibration waveforms, the processing todetermine downlink signature related signals related to the downlinksignature for the remote transceiver;

[0224] (c) transmitting the downlink signature related signals from theremote transceiver to the main transceiver using waveforms substantiallyconforming to the air interface standard; and

[0225] (d) determining the downlink signature of the remote transceiverfrom the downlink signature related signals received at the maintransceiver.

[0226] 2. The method of 1, wherein the at least one receive antennaelement are a plurality of receive antenna elements forming an array ofreceive a antenna elements, the number of elements in the array ofreceive antenna elements being the same as the number of antennaelements in the array of transmit antenna elements, the method furthercomprising:

[0227] (e) transmitting a set of one or more uplink calibrationwaveforms from the remote transceiver to the main transceiver, the setof downlink calibration waveforms substantially conforming to the airinterface standard;

[0228] (f) processing at the main transceiver the received antennasignals corresponding to the uplink calibration signals transmitted fromthe remote transceiver, the processing determining the uplink signaturefor the remote transceiver; and

[0229] (h) determining a calibration function for the main transceiverfrom the uplink and downlink signatures for the remote transceiver.

[0230] 3. In a wireless communication system comprising a maintransceiver and a remote transceiver capable of receiving signals fromand transmitting signals to the main transceiver, the main transceivercomprising an array of transmit antenna elements, and at least onereceive antenna element, each transmit antenna element being part of atransmit electronics chain for transmitting a transmit apparatus signalusing the transmit antenna element, and each receive antenna elementbeing part of a receiver apparatus chain for receiving a receivedantenna signal from the receive antenna element, the main transceiverand the remote transceiver designed for mutual communication usingwaveforms conforming to an air interface standard, a method forestimating the downlink signature for the remote transceiver, the methodcomprising:

[0231] (a) transmitting a set of one or more downlink calibrationwaveforms from the main transceiver via the transmit antenna array tothe remote transceiver, the set of downlink calibration waveformsdesigned to be robust to one or more of the set comprising frequencyoffset, phase noise, I/Q mismatch, and timing offset; signal using thetransmit antenna element, and each receive element being part of areceiver apparatus chain for receiving a received antenna signal fromthe receive antenna element, the main communication transceiver designedto transmit traffic waveforms, the main communication transceiver alsodesigned to transmit downlink calibration waveforms, a method forestimating the downlink signature for the remote transceiver, the methodcomprising:

[0232] (a) transmitting downlink calibration waveforms and trafficwaveforms from the main transceiver via the transmit antenna array tothe remote transceiver, the downlink calibration waveforms interspersedwith the traffic waveforms;

[0233] (b) determining at the remote transceiver whether the signalsreceived at the remote transceiver correspond to downlink calibrationwaveforms or to traffic waveforms;

[0234] (c) processing signals received at the remote transceiverdetermined in step (b) to correspond to downlink calibration waveforms,the processing to determine downlink signature related signals relatedto the downlink signature for the remote transceiver;

[0235] (d) processing signals received at the remote transceiverdetermined in step (b) to correspond to traffic waveforms, theprocessing to perform normal traffic functions;

[0236] (e) transmitting the downlink signature related signals from theremote transceiver to the main transceiver; and

[0237] (f) determining the downlink signature of the remote transceiverfrom the downlink signature related signals received at the maintransceiver.

[0238] 7. The method of 6, wherein the downlink calibration waveformsare transmitted during silent periods.

[0239] 8. The method of 6, wherein the downlink calibration waveformsare transmitted only after a number of idle waveforms are transmittedfrom the main transceiver.

[0240] 9. The method of 6, wherein the at least one receive antennaelement are a plurality of receive antenna elements forming an array ofreceive antenna elements, the number of elements in the array of receiveantenna elements being the same as the number of antenna elements in thearray of transmit antenna elements, the method further comprising:

[0241] (h) transmitting a set of one or more uplink calibrationwaveforms from the remote transceiver to the main transceiver, the setof downlink calibration waveforms;

[0242] (h) processing at the main transceiver the received antennasignals corresponding to the uplink calibration signals transmitted fromthe remote transceiver, the processing determining the uplink signaturefor the remote transceiver, and

[0243] (j) determining a calibration function for the main transceiverfrom the uplink and downlink signatures for the remote transceiver.

[0244] 10. The method of 6, wherein the downlink calibration waveformstransmitted from the main transceiver in step (a) are designed to berobust to one or more of the set comprising frequency offset, phasenoise, I/Q mismatch, and timing offset.

[0245] 11. In a wireless communication system comprising a maintransceiver and a remote transceiver capable of receiving signals fromand transmitting signals to the main transceiver, the main transceivercomprising an array of antenna elements and an array of receive antennaelements, each transmit antenna element being part of a transmitelectronics chain for transmitting a transmit apparatus signal using thetransmit antenna element, and each receive antenna element being part ofa receiver apparatus chain for receiving a received antenna signal fromthe receive antenna element, the number of elements in the array ofreceive antenna elements being the same as the number of antennaelements in the array of transmit antenna elements, the main transceivercomprises means for uplink adaptive smart antenna processing includinglinear uplink adaptive smart antenna processing according to an uplinkweight vector, and downlink adaptive smart antenna processing includinglinear downlink adaptive smart antenna processing according to adownlink weight vector, the main transceiver and the remote transceiverdesigned for mutual communication using waveforms conforming to an airinterface standard, the main transceiver further comprising means todetermine the uplink weight vector for the remote transceiver, a methodfor determining the downlink weight vector for the remote transceiver,the method comprising:

[0246] (a) transmitting a set of one or more downlink calibrationwaveforms from the main transceiver via the transmit antenna array tothe remote transceiver, the set of downlink calibration waveformssubstantially conforming to the air interface standard;

[0247] (b) processing the signals received at the remote transceivercorresponding to the downlink calibration waveforms, the processing todetermine downlink signature related signals related to the downlinksignature for the remote transceiver;

[0248] (c) transmitting the downlink signature related signals from theremote transceiver to the main transceiver;

[0249] (d) transmitting a set of one or more uplink calibrationwaveforms from the remote transceiver to the main transceiver;

[0250] (e) processing at the main transceiver the received antennasignals corresponding to the uplink calibration signals transmitted fromthe remote transceiver, the processing determining the uplink signaturefor the remote transceiver;

[0251] (f) determining the uplink weight vector for the remotetransceiver from any signals received at the main transceiver from theremote transceiver; and

[0252] (g) determining the downlink weight vector for the remotetransceiver from:

[0253] the determined uplink weight vector,

[0254] the determined uplink signature, and

[0255] the received antenna signals corresponding to the downlinksignature related signals received at the main transceiver.

[0256] 12. The method of 11, wherein the downlink weight determiningstep comprises:

[0257] (i) determining a calibration function for the remote transceiverfrom the determined uplink signature and the received antenna signalscorresponding to the downlink signature related signals received at themain transceiver, and from the downlink signature related signalsreceived at the main transceiver, and

[0258] (ii) determining the downlink weight vector from the determineduplink weight vector and the calibration function.

[0259] 13. In a wireless communication system comprising a maintransceiver and a plurality of remote transceivers each capable ofreceiving signals from and transmitting signals to the main transceiver,the main transceiver comprising an array of transmit antenna elements,and at least one receive antenna element, each transmit antenna elementbeing part of a transmit electronics chain for transmitting a transmitapparatus signal using the transmit antenna element, and each receiveantenna element being part of a receiver apparatus chain for receiving areceived antenna signal from the receive antenna element, a method forestimating the downlink signature for the remote transceiver, the methodcomprising:

[0260] (a) transmitting a set of one or more downlink calibrationwaveforms from the main transceiver via the transmit antenna array tothe remote transceivers;

[0261] (b) processing the signals received at each remote transceivercorresponding to the downlink calibration waveforms, the processing todetermine downlink signature related signals related to the downlinksignature for the remote transceiver;

[0262] (c) transmitting the downlink signature related signals from eachremote transceiver to the main transceiver using waveforms substantiallyconforming to the air interface standard;

[0263] (d) determining a downlink signature for each remote transceiverfrom the downlink signature related signals received at the maintransceiver from the remote transceiver; and

[0264] (e) combining the downlink signatures for the remote transceiversto determine a combined downlink signature.

[0265] 14. The method of 13, wherein the main transceiver and the remotetransceivers are designed for mutual communication using waveformsconforming to an air interface standard, and wherein each waveform inthe set of downlink calibration waveforms substantially conforms to theair interface standard.

[0266] 15. The method of 13, wherein the at least one receive antennaelement are a plurality of receive antenna elements forming an array ofreceive antenna elements, the number of elements in the array of receiveantenna elements being the same as the number of antenna elements in thearray of transmit antenna elements, the method further comprising:

[0267] (f) transmitting a set of one or more uplink calibrationwaveforms from each remote transceiver to the main transceiver;

[0268] (g) processing at the main transceiver the received antennasignals corresponding to the uplink calibration signals transmitted fromeach remote transceiver, the processing determining an uplink signaturefor each remote transceiver;

[0269] (h) combining the uplink signatures for the remote transceiversto determine a combined uplink signature; and

[0270] (i) determining a calibration function for the main transceiverfrom the uplink and downlink combined signatures.

[0271] 16. The method of 15, wherein the main transceiver comprisesmeans for uplink adaptive smart antenna processing including linearuplink adaptive smart antenna processing according to an uplink weightvector, and downlink adaptive smart antenna processing including lineardownlink adaptive smart antenna processing according to a downlinkweight vector, the method further comprising:

[0272] (k) determining at the main transceiver the uplink weight vectorfor receiving from, the subscriber unit by processing received antennasignals received while the remote transceiver is transmitting to themain transceiver; and

[0273] (l) determining at the main transceiver the downlink weight fortransmitting to the remote transceiver from the determined uplinkweights and the calibration factor.

[0274] 17. The method of 15, wherein the signature combining is carriedout by the principal component method.

[0275] 18. The method of 17, wherein each remote transmitter alsotransmits a remote transceiver received signal quality estimate to themain transceiver and wherein the signature combining is a weightedcombining, the weighting of the signature for each remote transceiverbeing the remote transceiver received signal quality estimate or theremote transceiver.

[0276] 19. The method of 15, wherein any component in a signatureestimate is discarded if it corresponds to a weak receive or transmitantenna element relative to the other antenna elements.

[0277] 20. The method of 2, wherein the uplink calibration signals areidle traffic waveforms.

[0278] 21. The method of 2, wherein the uplink calibration signals arethe downlink signature related signals.

[0279] 22. The method of 2, wherein the main transceiver comprises meansfor uplink adaptive smart antenna processing including linear uplinkadaptive smart antenna processing according to an uplink weight vector,and downlink adaptive smart antenna processing including linear downlinkadaptive smart antenna processing according to a downlink weight vector,the method further comprising:

[0280] (h) determining at the main transceiver the uplink weight vectorfor receiving from the subscriber unit by processing received antennasignals received while the remote transceiver is transmitting to themain transceiver; and

[0281] (j) determining at the main transceiver the downlink weight fortransmitting to the remote transceiver from the determined uplinkweights and the calibration factor.

[0282] 23. The method of 1, wherein the downlink signature is determinedin relation to a reference antenna element of the transmit antennaarray, and wherein the downlink calibration waveforms are selected sothat the signals transmitted from each transmit antenna elements aresubstantially orthogonal.

[0283] 24. The method of 23, wherein the downlink calibration waveformsare modulated constant modulus calibration signals selected so that thedot product of any two calibration signals transmitted from any twodistinct antenna elements of the transmit array is a pure tone.

[0284] 25. The method of 23, wherein the downlink calibration waveformscomprise combinations of M distinct modulated constant moduluscalibration signals, M being the number of antenna elements of theantenna array for which a downlink signature is being determined, eachcalibration signal comprising two segments, denoted a first segment anda second segment, respectively, the two segments being identically timedfor each calibration signal, wherein during the first segment timeinterval, a first set of linear combinations of the calibration signalsis transmitted from each of the antenna elements of the transmit array,and during the second segment time interval, a second set of linearcombinations of the calibration signals is transmitted from each of theantenna elements of the transmit array.

[0285] 26. The method of 23, wherein the signals transmitted from eachantenna element of the transmit array are modulated tone signals, thefrequencies of the tone signals from distinct arrays being distinct, thedownlink signature related signal determining processing step anddownlink signature determining step together comprising:

[0286] cross correlating the signals received at the remote transceiverwith each of the tone signals, and

[0287] normalizing the correlations with the signal transmitted from thereference element.

[0288] 27. The method of claim 23, wherein there are M antenna elements,the first set of linear combinations being a sum of M distinct tonesignals being transmitted from the reference antenna element, and noneof the tone signals being transmitted from the other transmit antennaelements, the frequencies of the tone of the distinct tone signals beingdistinct, the second set of linear combinations being a different one ofthe tone signals being transmitted from each of the antenna elements,the frequencies of the tones from distinct arrays being distinct,processing to determine the downlink signature related signals and thedownlink signature determining together comprising:

[0289] cross correlating the signals received during the first segmentat the remote transceiver with each of the first segment signalstransmitted by each antenna element to obtain first segmentcorrelations;

[0290] normalizing the first segment correlations with the first segmentcorrelation with the signal transmitted from the reference element, thenormalizing forming first segment normalized correlations;

[0291] cross correlating the signals received during the second segmentat the remote transceiver with each of the second segment signalstransmitted by each antenna element to obtain second segmentcorrelations;

[0292] normalizing the second segment correlations with the firstsegment correlation with the signal transmitted from the referenceelement, the normalizing forming second segment normalized correlations;and

[0293] dividing each the second segment normalized correlation with thecorresponding first segment normalized correlations to form the downlinksignature estimate components.

[0294] 28. The method of 1, where the downlink signature related signalscomprise the downlink signature for the remote transceiver.

[0295] 29. The method of 1, wherein the array of transmit antennaelements and the one or more receive antenna elements comprise commonantenna elements.

[0296] 30. The method of 1, wherein the downlink signature estimate isdetermined as the maximum likelihood estimate.

[0297] 31. The method of 1, wherein the communication system is acellular system comprising one or more base stations, each having one ormore subscriber units, and wherein the main transceiver is one of thebase stations.

[0298] 32. The method of 31, wherein the remote transceiver is asubscriber unit of the main transceiver.

[0299] 33. The method of 1, wherein the air interface standard is PHS.

[0300] 34. A wireless communication system comprising

[0301] (a) a main transceiver comprising:

[0302] (i) an array of transmit antenna elements, each transmit antennaelement being part of a transmit electronics chain for transmitting atransmit apparatus signal from the transmit antenna element,

[0303] (ii) one or more receive antenna elements, each receive antennaelement being part of a receive apparatus chain for receiving a receivedantenna signal from the receive antenna, and

[0304] (iii) one or more main transceiver signal processors forprocessing received antenna signals and for forming transmit apparatussignals; and

[0305] (b) a remote transceiver capable of receiving signals from andtransmitting signals to the main transceiver using waveforms conformingto an air interface standard, the remote transceiver comprising:

[0306] (i) a remote transceiver receiver including a remote transceiverreceive antenna for receiving remote transceiver received signals,

[0307] (i) a remote transceiver transmitter including a remotetransceiver transmit antenna for transmitting remote transceivertransmit signals to the main transceiver, and

[0308] (iii) one or more remote transceiver signal processors forprocessing remote transceiver received signals; and for forming remotetransceiver transmit signals,

[0309] wherein at least one of the main transceiver signal processors isprogrammed to:

[0310] transmit a set of downlink calibration waveforms from the maintransceiver via the transmit antenna array to the remote transceiver,the set of downlink calibration waveforms substantially conforming tothe air interface standard,

[0311] wherein at least one of the remote transceiver signal processorsis programmed to:

[0312] process the signals received corresponding to the transmitteddownlink calibration waveforms at the remote transceiver to determinedownlink signature related signals related to the downlink signature forthe remote transceiver;

[0313] transmit the downlink signature related signals from the remotetransceiver to the main transceiver using waveforms substantiallyconforming to the air interface standard, and

[0314] wherein at least one of the main transceiver signal processors isprogrammed to:

[0315] process the downlink signature related signals received at themain transceiver from the remote transceiver to determine the downlinksignature for the remote transceiver.

[0316] 35. The system of 34, wherein the at least one receive antennaelement are a plurality of receive antenna elements forming an array ofreceive antenna elements, the number of elements in the array of receiveantenna elements being the same as the number of antenna elements in thearray of transmit antenna elements, wherein at least one of the remotetransceiver signal processors is programmed to:

[0317] transmit a set of one or more uplink calibration waveforms to themain transceiver, and

[0318] wherein at least one of the main transceiver signal processors isprogrammed to:

[0319] process the received antenna signals corresponding to the uplinkcalibration waveforms transmitted from the remote transceiver, theprocessing determining the uplink signature for the remote transceiver;and

[0320] determine a calibration function for the main transceiver fromthe uplink and downlink signatures for the remote transceiver.

[0321] 36. The system of 35, wherein the main transceiver furthercomprises means for uplink adaptive smart antenna processing includinglinear uplink adaptive smart antenna processing according to an uplinkweight vector, and downlink adaptive smart antenna processing includinglinear downlink adaptive smart antenna processing according to adownlink weight vector, wherein at least one of the transceiver signalprocessors is programmed to:

[0322] determine the uplink weight vector for receiving from thesubscriber unit by processing received antenna signals received whilethe remote transceiver is transmitting to the main transceiver; and

[0323] determine the downlink weight for transmitting to the remotetransceiver from the uplink weights determined for the remotetransceiver and the calibration factor.

[0324] 37. A wireless communication system comprising

[0325] (a) a main transceiver comprising

[0326] (i) an array of transmit antenna elements, each transmit antennaelement being part of a transmit electronics chain for transmitting atransmit apparatus signal from the transmit antenna element,

[0327] (ii) an array of receive antenna elements, each receive antennaelement being part of a receive apparatus chain for receiving a receivedantenna signal from the receive antenna, the number of active elementsin the receive array being the same as the number of active elements inthe transmit array, and

[0328] (iii) one or more main transceiver signal processors forprocessing received antenna signals and for forming transmit apparatussignals; and

[0329] (b) a plurality of remote transceivers each capable of receivingsignals from and transmitting signals to the main transceiver, eachremote transceiver comprising:

[0330] (i) a remote transceiver receiver including a remote transceiverreceive antenna for receiving remote transceiver received signals,

[0331] (i) a remote transceiver transmitter including a remotetransceiver transmit antenna for transmitting remote transceivertransmit signals to the main transceiver, and

[0332] (iii) one or more remote transceiver signal processors forprocessing remote transceiver received signals; and for forming remotetransceiver transmit signals,

[0333] wherein at least one of the main transceiver signal processors isprogrammed to:

[0334] transmit a set of downlink calibration waveforms from the maintransceiver via the transmit antenna array to the plurality of remotetransceivers,

[0335] wherein at least one of each remote transceiver's remotetransceiver signal processors is programmed to:

[0336] process the signals received at the remote transceivercorresponding to the transmitted downlink calibration waveforms todetermine downlink signature related signals related to the downlinksignature for the remote transceiver,

[0337] transmit the downlink signature related signals from the remotetransceiver to the main transceiver using waveforms substantiallyconforming to the air interface standard, and

[0338] transmit a set of one or more uplink calibration signals from theremote transceiver to the main transceiver, and

[0339] wherein at least one of the main transceiver signal processors isprogrammed to:

[0340] process the downlink signature related signals received at themain transceiver from each remote transceiver to determine a downlinksignature for the remote transceiver,

[0341] process the received antenna signals corresponding to the uplinkcalibration signals from each remote transceiver to determine an uplinkcombined signature for the remote transceiver,

[0342] combine the downlink signatures for the transceivers to determinea downlink combined signature,

[0343] combine the uplink signatures for the transceivers to determinean uplink combined signature, and

[0344] determine a calibration factor for the main transceiver from thedownlink combined signature and from the uplink combined signature.

[0345] 38. The system of 35, wherein the uplink calibration signals areidle traffic waveforms.

[0346] 39. The system of 35, wherein the uplink calibration signals arethe downlink signature related signals.

[0347] 40. The system of 34, wherein the downlink signature isdetermined in relation to a reference antenna element of the transmitantenna array, and wherein the downlink calibration waveforms areselected so that the signals transmitted from each transmit antennaelements are substantially orthogonal.

[0348] 41. The system of 40, wherein the downlink calibration waveformsare designed to be robust to one or more of the set comprising frequencyoffset, phase noise, I/Q mismatch, and timing offset.

[0349] 42. The system of 41, wherein the downlink calibration waveformsare modulated constant modulus calibration signals selected so that thedot product of any two calibration signals transmitted from any twodistinct antenna elements of the transmit array is a pure tone.

[0350] 43. The system of 41, wherein the downlink calibration waveformscomprise combinations of M distinct modulated constant moduluscalibration signals, M being the number of antenna elements of theantenna array for which a downlink signature is being determined, eachcalibration signal comprising two segments, denoted a first segment anda second segment, respectively, the two segments being identically timedfor each calibration signal, wherein during the first segment timeinterval, a first set of linear combinations of the calibration signalsis transmitted from each of the antenna elements of the transmit array,and during the second segment time interval, a second set of linearcombinations of the calibration signals is transmitted from each of theantenna elements of the transmit array.

[0351] 44. The system of 42, wherein the signals transmitted from eachantenna element of the transmit array are modulated tone signals, thefrequencies of the tone signals from distinct arrays being distinct, thedownlink signature signals determining and the downlink signaturedetermining together comprising:

[0352] cross correlating the signals received at the remote transceiverwith each of the tone signals, and

[0353] normalizing the correlations with the signal transmitted from thereference element.

[0354] 45. The system of 41, wherein there are M antenna elements, thefirst set of linear combinations being a sum of M distinct tone signalsbeing transmitted from the reference antenna element, and none of thetone signals being transmitted from the other transmit antenna elements,the frequencies of the tone of the distinct tone. signals beingdistinct, the second set of linear combinations being a different one ofthe tone signals being transmitted from each of the antenna elements,the frequencies of the tones from distinct arrays being distinct, thedownlink signature signals determining and the downlink signaturedetermining together comprising:

[0355] cross correlating the signals received during the first segmentat the remote transceiver with each of the first segment signalstransmitted by each antenna element to obtain first segmentcorrelations;

[0356] normalizing the first segment correlations with the first segmentcorrelation with the signal transmitted from the reference element, thenormalizing forming first segment normalized correlations;

[0357] cross correlating the signals received during the second segmentat the remote transceiver with each of the second segment signalstransmitted by each antenna element to obtain second segmentcorrelations;

[0358] normalizing the second segment correlations with the firstsegment correlation with the signal transmitted from the referenceelement, the normalizing forming second segment normalized correlations;and

[0359] dividing each the second segment normalized correlation with thecorresponding first segment normalized correlations to form the downlinksignature estimate components.

[0360] 46. The system of 34, wherein the communication system is acellular system comprising one or more base stations, each having one ormore subscriber units, and wherein the main transceiver is one of thebase stations.

[0361] 47. The system of 34, wherein the remote transceiver is asubscriber unit of the main transceiver.

[0362] 48. The system of 34, wherein the air interface standard is PHS.

[0363] 49. The system of 34, where the downlink signature relatedsignals comprise the downlink signature for the remote transceiver.

[0364] 50. The system of 34, wherein the array of transmit antennaelements and the one or more receive antenna elements comprise commonantenna elements.

[0365] 51. The system of 34, wherein the downlink signature estimate isdetermined as the maximum likelihood estimate.

What is claimed is
 1. In a wireless communication system comprising amain transceiver and a remote transceiver capable of receiving signalsfrom and transmitting signals to the main transceiver, the maintransceiver comprising an array of transmit antenna elements, and atleast one receive antenna element, each transmit antenna element beingpart of a transmit electronics chain for transmitting a transmitapparatus signal using the transmit antenna element, and each receiveantenna element being part of a receiver apparatus chain for receiving areceived antenna signal from the receive antenna element, the maintransceiver and the remote transceiver designed for mutual communicationusing waveforms conforming to an air interface standard, a method forestimating the downlink signature for the remote transceiver, the methodcomprising: (a) transmitting a set of one or more downlink calibrationwaveforms from the main transceiver via the transmit antenna array tothe remote transceiver, the set of downlink calibration waveformssubstantially conforming to the air interface standard; (b) processingthe signals received at the remote transceiver corresponding to thedownlink calibration waveforms, the processing to determine downlinksignature related signals related to the downlink signature for theremote transceiver; (c) transmitting the downlink signature relatedsignals from the remote transceiver to the main transceiver usingwaveforms substantially conforming to the air interface standard; and(d) determining the downlink signature of the remote transceiver fromthe downlink signature related signals received at the main transceiver.2. The method of claim 1, wherein the at least one receive antennaelement are a plurality of receive antenna elements forming an array ofreceive antenna elements, the number of elements in the array of receiveantenna elements being the same as the number of antenna elements in thearray of transmit antenna elements, the method further comprising: (e)transmitting a set of one or more uplink calibration waveforms from theremote transceiver to the main transceiver, the set of downlinkcalibration waveforms substantially conforming to the air interfacestandard; (f) processing at the main transceiver the received antennasignals corresponding to the uplink calibration signals transmitted fromthe remote transceiver, the processing determining the uplink signaturefor the remote transceiver; and (h) determining a calibration functionfor the main transceiver from the uplink and downlink signatures for theremote transceiver.
 3. In a wireless communication system comprising amain transceiver and a remote transceiver capable of receiving signalsfrom and transmitting signals to the main transceiver, the maintransceiver comprising an array of transmit antenna elements, and atleast one receive antenna element, each transmit antenna element beingpart of a transmit electronics chain for transmitting a transmitapparatus signal using the transmit antenna element, and each receiveantenna element being part of a receiver apparatus chain for receiving areceived antenna signal from the receive antenna element, the maintransceiver and the remote transceiver designed for mutual communicationusing waveforms conforming to an air interface standard, a method forestimating the downlink signature for the remote transceiver, the methodcomprising: (a) transmitting a set of one or more downlink calibrationwaveforms from the main transceiver via the transmit antenna array tothe remote transceiver, the set of downlink calibration waveformsdesigned to be robust to one or more of the set comprising frequencyoffset, phase noise, I/Q mismatch, and timing offset; (b) processing thesignals received at the remote transceiver corresponding to the downlinkcalibration waveforms, the processing to determine downlink signaturerelated signals related to the downlink signature for the remotetransceiver; (c) transmitting the downlink signature related signalsfrom the remote transceiver to the main transceiver; and (d) determiningthe downlink signature of the remote transceiver from the downlinksignature related signals received at the main transceiver.
 4. Themethod of claim 3, wherein the at least one receive antenna element area plurality of receive antenna elements forming an array of receiveantenna elements, the number of elements in the array of receive antennaelements being the same as the number of antenna elements in the arrayof transmit antenna elements, the method further comprising: (e)transmitting a set of one or more uplink calibration waveforms from theremote transceiver to the main transceiver, the set of downlinkcalibration waveforms; (f) processing at the main transceiver thereceived antenna signals corresponding to the uplink calibration signalstransmitted from the remote transceiver, the processing determining theuplink signature for the remote transceiver; and (g) determining acalibration function for the main transceiver from the uplink anddownlink signatures for the remote transceiver.
 5. The method of claim3, wherein each of the set of downlink calibration waveforms conforms tothe air interface standard.
 6. In a wireless communication systemcomprising a main transceiver and a remote transceiver capable ofreceiving signals from and transmitting signals to the main transceiver,the main transceiver comprising an array of transmit antenna elements,and at least one receive antenna element, each transmit antenna elementbeing part of a transmit electronics chain for transmitting a transmitapparatus signal using the transmit antenna element, and each receiveelement being part of a receiver apparatus chain for receiving areceived antenna signal from the receive antenna element, the maincommunication transceiver designed to transmit traffic waveforms, themain communication transceiver also designed to transmit downlinkcalibration waveforms, a method for estimating the downlink signaturefor the remote transceiver, the method comprising: (a) transmittingdownlink calibration waveforms and traffic waveforms from the maintransceiver via the transmit antenna array to the remote transceiver,the downlink calibration waveforms interspersed with the trafficwaveforms; (b) determining at the remote transceiver whether the signalsreceived at the remote transceiver correspond to downlink calibrationwaveforms or to traffic waveforms; (c) processing signals received atthe remote transceiver determined in step (b) to correspond to downlinkcalibration waveforms, the processing to determine downlink signaturerelated signals related to the downlink signature for the remotetransceiver; (d) processing signals received at the remote transceiverdetermined in step (b) to correspond to traffic waveforms, theprocessing to perform normal traffic functions; (e) transmitting thedownlink signature related signals from the remote transceiver to themain transceiver; and (f) determining the downlink signature of theremote transceiver from the downlink signature related signals receivedat the main transceiver.
 7. The method of claim 6, wherein the downlinkcalibration waveforms are transmitted during silent periods.
 8. Themethod of claim 6, wherein the downlink calibration waveforms aretransmitted only after a number of idle waveforms are transmitted fromthe main transceiver.